HOMER — Motif Analysis and Peak Annotation

HOMER — Motif Analysis and Peak Annotation

Overview

HOMER (Hypergeometric Optimization of Motif EnRichment) is a suite of Perl/C++ tools for analyzing genomic regulatory elements. Its two primary commands are findMotifsGenome.pl, which performs de novo motif discovery and known motif enrichment against JASPAR/HOMER databases, and annotatePeaks.pl, which maps each peak to the nearest gene, distance to TSS, and genomic feature class (promoter, intron, intergenic, repeat). HOMER takes BED-format peak files from MACS3 or similar peak callers and a reference genome assembly as input, and outputs HTML/text reports ranking enriched motifs by p-value and fold enrichment over a matched background.

When to Use

• Identifying which transcription factors are bound in a ChIP-seq peak set by enriching their known motifs from JASPAR or the HOMER motif library
• Discovering novel sequence motifs de novo in open chromatin regions from ATAC-seq without prior knowledge of the binding TF
• Comparing motif landscapes between two conditions (e.g., treated vs. untreated peak sets) by running HOMER with one set as target and the other as background
• Annotating genomic peaks with nearest genes and distance to TSS for downstream functional analysis or integration with DESeq2 results
• Validating ChIP-seq experiment quality: a successful pull-down should show the target TF's canonical motif as the top hit
• Use macs3-peak-calling first to generate the peak BED files that serve as input to HOMER
• Use jaspar-database to cross-reference HOMER-discovered motifs with JASPAR IDs and additional TF metadata
• Use MEME-CHIP (web or local) when you need a more probabilistic ZOOPS/TCM model or the MEME Suite ecosystem
• Use AME (part of MEME Suite) as a faster alternative for known motif scanning without de novo discovery

Prerequisites

• Software: HOMER (Perl + compiled binaries), conda or manual install
• Genomes: must download genome sequence via installGenome.pl after HOMER install
• Input: BED file of peaks (at minimum: chr, start, end columns); ideally summit-centered peaks from MACS3
• Python packages (for parsing/visualization): pandas, matplotlib, seaborn

Check before installing: The tool may already be available in the current environment (e.g., inside a pixi / conda env). Run command -v findMotifsGenome.pl first and skip the install commands below if it returns a path. When running inside a pixi project, invoke the tool via pixi run findMotifsGenome.pl rather than bare findMotifsGenome.pl.

# Install HOMER via conda (recommended — handles Perl dependencies)
conda install -c bioconda homer

# Verify installation
findMotifsGenome.pl 2>&1 | head -3
# Usage: findMotifsGenome.pl <peak/BED file> <genome> <output directory> [options]

annotatePeaks.pl 2>&1 | head -3
# Usage: annotatePeaks.pl <peak/BED file> <genome> [options]

# Install reference genomes (downloads 2-way masker + sequence; ~3–10 GB each)
installGenome.pl hg38