Imported from FridrichMethod/awesome-skills (skills/clip-alignment/SKILL.md). Install upstream with npx skills add FridrichMethod/awesome-skills --skill clip-alignment. Copyright stays with the author.

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

Methodology evolves; verify the current ENCODE eCLIP pipeline (encodeproject.org/eclip) before locking parameters. The ENCODE eCLIP SOP v2.2 pins STAR 2.5.2b (the earlier v1 SOP used 2.4.0i); 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).

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 (Zhang & Xing 2017) rescues additional peaks in repeat regions (operationally ~10-30% more, dataset-dependent). 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

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

Operational rule: For ENCODE-comparable analysis, use STAR 2.5.2b (ENCODE eCLIP SOP v2.2) 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

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)
• West C et al 2023 Wellcome Open Res 8:286 (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