Clip alignment

Version Compatibility

Reference examples tested with: STAR 2.7.11b+, bowtie2 2.5.3+, HISAT2 2.2.1+, samtools 1.19+, CLAM 1.2+, umi_tools 1.1.5+.

Before using code patterns, verify installed versions match. If versions differ:

• CLI: <tool> --version then <tool> --help to confirm flags

If code throws unexpected errors, introspect the installed binary (<tool> -h) and adapt the example to match the actual CLI rather than retrying.

CLIP-seq Alignment

"Align preprocessed CLIP reads to genome with crosslink-preserving parameters" -> Map UMI-extracted, adapter-trimmed reads to the genome (NOT transcriptome) with end-to-end alignment, strict mismatch ceiling, and unique-mapper-only filtering by default. The 5' end of the read (R2 5' in paired-end eCLIP; R1 5' in iCLIP) carries the reverse-transcriptase truncation = crosslink site -1; any soft-clipping or 5' trimming during alignment destroys nucleotide resolution.

• CLI (eCLIP / iCLIP / iCLIP2, ENCODE pattern): STAR --runMode alignReads --genomeDir STAR_index --readFilesIn R1.trim.fq.gz R2.trim.fq.gz --readFilesCommand zcat --outFilterType BySJout --outFilterMultimapNmax 1 --alignEndsType EndToEnd --outFilterMismatchNoverReadLmax 0.04 --outSAMtype BAM SortedByCoordinate --outSAMattributes All --outFileNamePrefix sample_
• CLI (PAR-CLIP, mismatch ceiling raised for T->C signal): same as above but --outFilterMismatchNoverReadLmax 0.07
• CLI (low memory, no splicing): bowtie2 -x genome_index -U R1.trim.fq.gz --very-sensitive -p 8 | samtools view -bS - | samtools sort -o aligned.bam
• CLI (repeat-binding RBP, multi-mapper rescue): STAR ... --outFilterMultimapNmax 100 --outSAMmultNmax -1 then process with CLAM (see repeat-element section)

End-to-end alignment is non-negotiable. --alignEndsType Local (STAR default for some pipelines) soft-clips low-quality 5' bases and discards exactly the truncation signal. Mismatch ceiling 0.04 (4% of read length) excludes most sequencing-error reads; 0.07 is the PAR-CLIP override to retain T->C reads.

Algorithmic Taxonomy

Aligner	Splice-aware	Memory (human)	CLIP-suitable	Strength	Fails when
STAR	Yes	~30 GB peak (sjdbOverhang 100)	Yes (ENCODE eCLIP standard)	Splice-aware, fast on deep libraries, excellent multi-mapper logs	RAM hungry; small clusters or laptops cannot run human genome index
HISAT2	Yes	~8 GB	Yes (good alternative when STAR memory infeasible)	Low memory; comparable splice accuracy	Slightly lower multi-mapper precision; smaller community adoption for CLIP
bowtie2	No (no splice)	~3 GB	Limited (loses intronic and read-through reads at splice junctions)	Fast, low memory, mature	Misses spliced reads; not suitable for mRNA-binding RBPs (PTBP1, U2AF2)
bwa-mem2	No	~30 GB index	Not recommended	Fast for DNA; no splice support	Same splicing issue as bowtie2; no CLIP-specific advantage
chromap	No	~5 GB	Not recommended for CLIP	Very fast for ATAC/ChIP	No splice support; pre-applies fragment shift inappropriate for CLIP
novoalign	Yes (splice with -X)	~10 GB	Acceptable	Old standard; some labs still use	Commercial; community moved to STAR/HISAT2
Salmon (transcriptome)	N/A	low	Not suitable	Pseudo-alignment	Discards intronic and unannotated reads, both common in CLIP

Methodology evolves; verify the current ENCODE eCLIP pipeline (encodeproject.org/eclip) before locking parameters. As of 2025, ENCODE 4 still uses STAR 2.4.0j (legacy version pinning); modern reanalyses use STAR 2.7.x with the same parameter set.

Critical Choice: Unique-Mapper-Only vs Multi-Mapper Rescue

Two valid alignment strategies depend on the RBP biology:

Strategy A -- Unique-mapper-only (default, ENCODE standard): --outFilterMultimapNmax 1 discards every read that maps to more than one genomic location. Loses 5-15% of reads but produces unambiguous positions for downstream peak calling and crosslink-site detection.

Strategy B -- Multi-mapper rescue (for repeat-binding RBPs): --outFilterMultimapNmax 100 --outSAMmultNmax -1 retains up to 100 alignments per read; downstream EM-based assignment (CLAM, Xinglab) probabilistically allocates them. Required for RBPs that bind Alu, LINE-1, LTR, or other repetitive elements (e.g., MATR3, ILF3, FUS at LINE-1, HNRNPK at SINEs, PUM2 in some repeat contexts).

RBP class	Strategy	Why
Splicing factors (PTBP1, U2AF2, RBFOX, SRSF1)	A (unique)	Bind defined intronic/exonic motifs, not repeats
3' UTR stability (HuR, PUM2, AUF1)	A (unique)	Binding sites are usually in unique 3' UTR sequence
Ribosomal proteins (RPS19, RPL35A)	A (unique)	Bind mRNA bodies and snoRNAs in unique sequence
Translation initiation (EIF3J, EIF2S2)	A (unique)	Bind 5' UTRs and snoRNAs
Repeat / TE binders (MATR3, ZFP36 isoforms, HNRNPK, LINE-1 ORFs)	B (multi-mapper)	Genuine biology is in repeat regions
Y-RNA / 7SK / vault RNAs (TROVE2, LARP7)	A (unique) but with --outFilterMultimapNmax 50 for the ncRNA itself	These small RNAs have a few unique copies; pure unique discards them
Mitochondrial RBPs (FASTKD2, LRPPRC, TFAM)	A (unique) with chrM retained	chrM has unique sequence but must not be excluded by blacklists
Histone mRNA (SLBP)	A (unique) but DO NOT pre-filter rRNA index that includes histone	Replication-dependent histone genes are repetitive in some indices

Per-Aligner Failure Modes

STAR -- Local alignment soft-clips the truncation site

Trigger: Pipeline copied from RNA-seq tutorial uses --alignEndsType Local (or omits the flag, letting STAR default).

Mechanism: Local alignment soft-clips up to 12% of the read end if scoring improves. In CLIP, the 5' read end is the truncation = CL site -1 base; if it carries even one mismatched base from RT errors, STAR soft-clips it.

Symptom: Crosslink-site density looks "smoothed"; PureCLIP / CTK CITS detect 5-30% fewer single-nt sites than expected; peak boundaries look fuzzy.

Fix: Always pass --alignEndsType EndToEnd. To confirm: samtools view dedup.bam | awk '{ if ($6 ~ /S/) print }' | wc -l should report < 1% of reads with soft-clip CIGAR operations.

STAR -- Mismatch ceiling discards PAR-CLIP signal

Trigger: PAR-CLIP library aligned with --outFilterMismatchNoverReadLmax 0.04 (the iCLIP/eCLIP default).

Mechanism: PAR-CLIP T->C conversion rate is 20-50% of T positions in crosslinked reads. For a 30 nt read with 8 Ts and 50% conversion, ~4 T->C mismatches occur (13% of read length). The 4% mismatch ceiling drops these reads.

Symptom: 40-70% read loss at alignment for PAR-CLIP; downstream PARalyzer / wavClusteR find few clusters.

Fix: For PAR-CLIP only, set --outFilterMismatchNoverReadLmax 0.07 (7%). Verify post-alignment: samtools view dedup.bam | awk '{ for(i=12;i<=NF;i++) if($i ~ /^MD:/) print $i }' | head should show many T->C-indicating MD tags.

STAR -- Multi-mappers silently discarded when needed

Trigger: Repeat-binding RBP (MATR3, LINE-1 binders) aligned with default --outFilterMultimapNmax 1.

Mechanism: Reads mapping to more than one Alu/LINE/LTR instance are removed; 10-30% of true binding sites are lost.

Symptom: Compared to published RBP literature for repeat binders, the analysis peak count is 3-10x lower; repeat-overlap fraction is < 5% when literature suggests 15-30%.

Fix: Raise --outFilterMultimapNmax to 50-100; emit all alignments with --outSAMmultNmax -1; downstream use CLAM (Xinglab) to probabilistically assign multi-mappers to a single best location via expectation-maximization. Or restrict to a non-repeat analysis and note the limitation.

bowtie2 -- Splice junctions missed silently

Trigger: Used bowtie2 instead of STAR for an mRNA-binding RBP (typical of PTBP1, U2AF2 intronic binding studies); BAM looks normal but introns/exons map poorly.

Mechanism: bowtie2 has no splice model; reads spanning exon-intron junctions soft-clip 5-20 nt or fail to align entirely.

Symptom: Lower read recovery (~70-80% vs STAR ~90-95%); peaks at splice sites under-called; intronic peaks over-represented (reads that would have aligned across an exon now align fully in the intron upstream).

Fix: Use STAR or HISAT2 for any RBP that binds mRNA, pre-mRNA, or splice signals. Reserve bowtie2 for non-coding RNA targets (snoRNA, 7SK, Y RNA) where short reads do not span junctions.

STAR -- Memory exhausted on small machine

Trigger: Human genome index loaded into < 32 GB RAM machine.

Mechanism: STAR's suffix-array index requires ~30 GB RAM for human/mouse. Smaller machines crash or swap.

Symptom: "STAR EXITED" with bus error; or alignment takes > 24h on a single sample.

Fix: Switch to HISAT2 (~8 GB peak); or build a STAR index with --genomeSAsparseD 2 (halves RAM at the cost of ~30% alignment slowdown); or use a public cluster with > 64 GB. Do NOT use bowtie2 as a memory workaround for splice-aware needs.

Read-2 5' end trimmed inadvertently

Trigger: A --clip5pNbases flag carried over from RNA-seq quality trimming; or a fastp --trim_front2 5 step in preprocessing.

Mechanism: Removes the eCLIP truncation base on R2 5'.

Symptom: Crosslink sites called from R2 5' positions cluster artificially at uniform offsets across genome; motif enrichment around crosslink sites collapses.

Fix: Verify the BAM with samtools view dedup.bam | awk '$2 ~ /83|163/ { print $4 }' (R2 5' positions on minus and plus strand) and confirm they map to the EXPECTED truncation sites, not shifted by 5 nt.

ENCODE 4 eCLIP STAR Parameters (Reference)

STAR --runMode alignReads \
    --runThreadN 16 \
    --genomeDir /path/to/STAR_hg38_index \
    --genomeLoad NoSharedMemory \
    --readFilesIn R1.trim.fq.gz R2.trim.fq.gz \
    --readFilesCommand zcat \
    --outFilterType BySJout \
    --outFilterMultimapNmax 1 \
    --alignEndsType EndToEnd \
    --outFilterMismatchNoverReadLmax 0.04 \
    --outSAMtype BAM SortedByCoordinate \
    --outSAMattributes All \
    --outFileNamePrefix sample_ \
    --outFilterScoreMinOverLread 0.66 \
    --outFilterMatchNminOverLread 0.66

The --outFilterScoreMinOverLread 0.66 / --outFilterMatchNminOverLread 0.66 (matched score / matched bases >= 66% of read length) are ENCODE-specific stringency added on top of the mismatch ceiling. These two flags are why ENCODE eCLIP BAMs are smaller than naive STAR output.

Post-Alignment Filtering

# Sort and index
samtools index sample_Aligned.sortedByCoord.out.bam

# MAPQ filter (255 in STAR = unique; bowtie2 uses different scheme)
samtools view -b -q 10 sample_Aligned.sortedByCoord.out.bam > sample_q10.bam
samtools index sample_q10.bam

# UMI deduplication (see clip-preprocessing for `--method=unique` rationale)
umi_tools dedup \
    --stdin=sample_q10.bam \
    --stdout=sample_dedup.bam \
    --method=unique \
    --paired \
    --log=sample_dedup.log
samtools index sample_dedup.bam

MAPQ thresholds differ by aligner:

• STAR: 255 = uniquely mapped (== --outFilterMultimapNmax 1 already filtered); lower MAPQ values mean multi-mappers
• bowtie2: 42 = unique; 0-1 = multi-mapper; intermediate = ambiguous
• HISAT2: 60 = unique; similar to bowtie2 scheme but tool-specific

For STAR -q 10 is conventional but redundant if --outFilterMultimapNmax 1 was already set. For bowtie2/HISAT2, -q 30 is a stricter unique-mapper proxy.

Multi-Mapper Rescue with CLAM (Repeat-Binding RBPs)

# 1. Re-align permitting multi-mappers
STAR --runMode alignReads \
    --genomeDir STAR_index --readFilesIn R1.fq.gz R2.fq.gz \
    --readFilesCommand zcat --outFilterMultimapNmax 100 \
    --outSAMmultNmax -1 --alignEndsType EndToEnd \
    --outSAMtype BAM SortedByCoordinate --outFileNamePrefix mm_

samtools index mm_Aligned.sortedByCoord.out.bam

# 2. CLAM preprocessing splits unique vs multi-mapper reads
CLAM preprocessor -i mm_Aligned.sortedByCoord.out.bam -o clam_out/ --read-tagger-method median

# 3. EM-based multi-mapper realignment
CLAM realigner -i clam_out/unique.sorted.bam -o clam_out/ --winsize 50 --max-tags 0

# 4. Downstream peakcaller (CLAM peakcaller is a wrapper for Piranha+EM)
CLAM peakcaller -i clam_out/unique.sorted.bam clam_out/realigned.sorted.bam \
    -o clam_peaks.bed -p 8 --gtf gencode.v38.annotation.gtf

CLAM rescues 10-30% additional peaks in repeat regions (Zhang & Xing 2017). It is the only EM-based multi-mapper solution actively maintained for CLIP as of 2025.

HISAT2 Low-Memory Alternative

hisat2 --rna-strandness FR --no-softclip \
    -p 8 -x hisat2_grch38_index \
    -1 R1.trim.fq.gz -2 R2.trim.fq.gz \
    --no-unal \
    --score-min L,0,-0.2 \
    -S sample.sam 2> hisat2.log

samtools view -bS sample.sam | samtools sort -o sample.bam -
samtools index sample.bam

--no-softclip is the HISAT2 equivalent of STAR's --alignEndsType EndToEnd. --score-min L,0,-0.2 is roughly equivalent to STAR's --outFilterMismatchNoverReadLmax 0.04 (penalty scales linearly with read length, with -0.2 per mismatch slope).

Decision Tree by Use Case

Scenario	Recommended aligner + parameters	Why
eCLIP, ENCODE-comparable, 32+ GB RAM	STAR ENCODE pattern (above)	Reproducible against ENCODE peak calls
iCLIP / iCLIP2, single-end	STAR same params, --readFilesIn R1.fq.gz	Single-end variant of ENCODE block
PAR-CLIP	STAR with --outFilterMismatchNoverReadLmax 0.07	T->C is signal, not error
Repeat-binding RBP (MATR3, LINE-1 binders)	STAR --outFilterMultimapNmax 100 --outSAMmultNmax -1 + CLAM	EM rescue of multi-mappers
Low memory (< 16 GB)	HISAT2 with --no-softclip	STAR index unloadable
Bacterial / yeast (small genome, no splicing)	bowtie2 --very-sensitive	Splice-awareness wasted
snoRNA-only / 7SK-only analysis	bowtie2 with custom index	Avoid genome-scale splicing tangle
Allele-specific CLIP	STAR ENCODE + WASP filter (--waspOutputMode SAMtag)	Reference-allele mapping bias must be removed
Long-read CLIP (dirCLIP, nanopore)	minimap2 -ax splice -uf -k14	STAR cannot align long reads

Allele-Specific Alignment with WASP

CLIP-seq inherits reference-allele mapping bias from RNA-seq. For allele-specific binding analyses (BEAPR, ASPRIN), STAR's WASP integration removes reads where the alternative-allele version of the read would not have aligned at the same position.

# STAR with WASP filter; --varVCFfile is the heterozygous SNP VCF
STAR --runMode alignReads \
    --genomeDir STAR_index --readFilesIn R1.fq.gz R2.fq.gz --readFilesCommand zcat \
    --alignEndsType EndToEnd --outFilterMultimapNmax 1 --outFilterMismatchNoverReadLmax 0.04 \
    --outSAMtype BAM SortedByCoordinate \
    --varVCFfile sample.het.vcf.gz \
    --waspOutputMode SAMtag \
    --outSAMattributes vA vG vW \
    --outFileNamePrefix wasp_

samtools view -b -e '[vW]==1' wasp_Aligned.sortedByCoord.out.bam > wasp_pass.bam

WASP filtering is MANDATORY for allele-specific binding analyses; reference-allele bias inflates REF allele frequency 1-5% in unfiltered CLIP data.

Reconciliation: When Alignment Outputs Disagree

Pattern	Likely cause	Action
STAR retains more reads than bowtie2	STAR handles splice junctions; bowtie2 fails on spliced reads	Trust STAR for mRNA-binding RBPs
Read count after STAR < expected	--outFilterMultimapNmax 1 discarding multi-mappers	If RBP binds repeats, switch to multi-mapper mode + CLAM
Crosslink-site density looks smoothed	Soft-clip ON (alignEndsType Local)	Re-align with --alignEndsType EndToEnd
40-70% read loss in PAR-CLIP	T->C mismatches exceed 4% ceiling	Raise --outFilterMismatchNoverReadLmax to 0.07
HISAT2 calls peaks STAR misses	HISAT2 lower-stringency soft-clip behaviour	Re-run HISAT2 with --no-softclip; differences should narrow to < 5%
Two STAR versions (2.4 vs 2.7) give different BAMs	Index sjdb format changed; parameter defaults drift	Pin STAR version for cross-study comparison; document

Operational rule: For ENCODE-comparable analysis, use STAR 2.4.0j or 2.7.x with the ENCODE eCLIP parameter block; use --alignEndsType EndToEnd; use --outFilterMultimapNmax 1 unless the biology requires multi-mappers (then add CLAM); UMI-dedupe with umi_tools dedup --method=unique. Document any deviation in methods.

Common Errors

Error / symptom	Cause	Solution
STAR ERROR ... Could not allocate memory	Index too large for RAM	Switch to HISAT2; or rebuild STAR index with --genomeSAsparseD 2
samtools sort: not enough memory after STAR	sort -m default 768M too small	samtools sort -m 4G -@ 8
Crosslink density at uniform 5 nt offsets across genome	R2 5' trimmed by mistake	Recheck preprocessing; cutadapt -g and fastp --trim_front2 are banned for CLIP
90% of reads MAPQ < 10	Genome index built for different species	Verify with samtools view -h sample.bam | head against expected chromosome names
HISAT2 splice junctions miss-detected	Used --no-splice (HISAT2 splice OFF)	Remove --no-splice for mRNA studies
umi_tools dedup OOM	Too many UMIs at one position; deep library	Switch to --method=unique (less RAM than directional)
Multi-mapper count after STAR --outFilterMultimapNmax 100 = 0	--outSAMmultNmax not set	Add --outSAMmultNmax -1 to emit all alignments
Reads MAPQ 0 dominate after bowtie2	--very-sensitive ON but multi-mappers retained	Add samtools view -q 30 post-filter
CLAM EM never converges	Background regions empty; sparse coverage	Lower --max-tags to 5; provide gene model GTF; verify multi-mapper BAM exists

References

• Van Nostrand EL et al 2016 Nat Methods 13:508 (eCLIP / ENCODE alignment parameter block)
• Konig J et al 2010 Nat Struct Mol Biol 17:909 (iCLIP truncation-as-CL principle)
• Dobin A et al 2013 Bioinformatics 29:15 (STAR aligner)
• Kim D et al 2019 Nat Biotechnol 37:907 (HISAT2)
• Langmead B & Salzberg SL 2012 Nat Methods 9:357 (bowtie2)
• Zhang Z & Xing Y 2017 Nucleic Acids Res 45:9260 (CLAM multi-mapper assignment)
• van de Geijn B et al 2015 Nat Methods 12:1061 (WASP allele-specific alignment)
• Hafner M et al 2010 Cell 141:129 (PAR-CLIP T->C mismatch tolerance need)
• Chakrabarti AM et al 2023 Genome Biol 24:235 (nf-core/clipseq alignment defaults)

Related Skills

• clip-seq/clip-preprocessing - UMI extraction and adapter trimming before alignment
• clip-seq/clip-qc - Post-alignment QC (library complexity, read distribution, FRiP)
• clip-seq/crosslink-site-detection - Why the 5' end must be preserved
• clip-seq/clip-peak-calling - Downstream peak calling on the dedup BAM
• read-alignment/star-alignment - General STAR usage and indexing
• read-alignment/bowtie2-alignment - General bowtie2 usage
• alignment-files/duplicate-handling - Picard MarkDuplicates as UMI-less fallback
• single-cell/scatac-analysis - Cross-reference for single-cell variants