DELLY2: Structural variant discovery by integrated paired-end and split-read analysis
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Delly is an integrated structural variant (SV) prediction method that can discover, genotype and visualize deletions, tandem duplications, inversions and translocations at single-nucleotide resolution in short-read massively parallel sequencing data. It uses paired-ends, split-reads and read-depth to sensitively and accurately delineate genomic rearrangements throughout the genome. Structural variants can be visualized using Delly-maze and Delly-suave.

Installing Delly

The easiest way to get Delly is to download a statically linked binary from the Delly github release page or to download Delly from Bioconda. You can also build Delly from source using a recursive clone and make.

git clone --recursive

cd delly/

make all

There is a Delly discussion group delly-users for usage and installation questions and a dockerized delly.

Delly multi-threading mode

Delly supports parallel computing using the OpenMP API (

make PARALLEL=1 -B src/delly

There is also a statically linked, multi-threaded binary for Linux 64-bit available under releases.

You can set the number of threads using the environment variable OMP_NUM_THREADS.


Delly primarily parallelizes on the sample level. Hence, OMP_NUM_THREADS should be always smaller or equal to the number of input samples.

Running Delly

Delly needs a sorted, indexed and duplicate marked bam file for every input sample. An indexed reference genome is required to identify split-reads. The output is in BCF format with a csi index. Delly supports germline and somatic SV discovery, genotyping and filtering. Because of that, Delly has been modularized and common workflows for germline and somatic SV calling are outlined below. If you do need VCF output you need a recent version of BCFtools for file conversion .

delly call -x hg19.excl -o delly.bcf -g hg19.fa input.bam

bcftools view delly.bcf > delly.vcf

Somatic SV calling

  • At least one tumor sample and a matched control sample are required for SV discovery

delly call -x hg19.excl -o t1.bcf -g hg19.fa tumor1.bam control1.bam

  • Somatic pre-filtering requires a tab-delimited sample description file where the first column is the sample id (as in the VCF/BCF file) and the second column is either tumor or control.

delly filter -f somatic -o t1.pre.bcf -s samples.tsv t1.bcf

  • Genotype pre-filtered somatic sites across a larger panel of control samples to efficiently filter false postives and germline SVs. For performance reasons, this can be run in parallel for each sample of the control panel and you may want to combine multiple pre-filtered somatic site lists from multiple tumor samples.

delly call -g hg19.fa -v t1.pre.bcf -o geno.bcf -x hg19.excl tumor1.bam control1.bam ... controlN.bam

  • Post-filter for somatic SVs using all control samples.

delly filter -f somatic -o t1.somatic.bcf -s samples.tsv geno.bcf

Germline SV calling

  • SV calling is done by sample for high-coverage genomes or in small batches for low-coverage genomes

delly call -g hg19.fa -o s1.bcf -x hg19.excl sample1.bam

  • Merge SV sites into a unified site list

delly merge -o sites.bcf s1.bcf s2.bcf ... sN.bcf

  • Genotype this merged SV site list across all samples. This can be run in parallel for each sample.

delly call -g hg19.fa -v sites.bcf -o s1.geno.bcf -x hg19.excl s1.bam

delly call -g hg19.fa -v sites.bcf -o sN.geno.bcf -x hg19.excl sN.bam

  • Merge all genotyped samples to get a single VCF/BCF using bcftools merge

bcftools merge -m id -O b -o merged.bcf s1.geno.bcf s2.geno.bcf ... sN.geno.bcf

  • Apply the germline SV filter which requires at least 20 unrelated samples

delly filter -f germline -o germline.bcf merged.bcf


  • What is the smallest SV size Delly can call?
    This depends on the sharpness of the insert size distribution. For an insert size of 200-300bp with a 20-30bp standard deviation, Delly starts to call reliable SVs >=300bp. Delly also supports calling of small InDels using soft-clipped reads only (-i option). In this mode the smallest SV size called is 15bp.

  • Is Delly read-group aware?
    Yes. If you want to estimate separate insert size distributions for every read-group then use the -e option.

  • Can Delly be used on a non-diploid genome?
    Yes and no. The SV site discovery works for any ploidy. However, Delly's genotyping model assumes diploidy (hom. reference, het. and hom. alternative).

  • How do I run Delly if I have multiple different libraries/bam files for a single sample?
    Merge these BAMs using tools such as Picard and tag each library with a unique ReadGroup. If you have a sample with multiple read-groups please run delly with the -e option.

  • Delly is running too slowly what can I do?
    You should exclude telomere and centromere regions and also all unplaced contigs. Delly ships with such an exclude list for human and mouse samples. In addition, you can filter input reads more stringently using -q 20 and -s 15. Small InDel calling takes more time (-i option), leave it out if you are only interested in large SVs (>=300bp).

  • Are non-unique alignments, multi-mappings and/or multiple split-read alignments allowed?
    Delly expects two alignment records in the bam file for every paired-end, one for the first and one for the second read. Multiple split-read alignment records of a given read are allowed if and only if one of them (e.g. the longest split alignment) is a primary alignment whereas all others are marked as secondary or supplementary (flag 0x0100 or flag 0x0800). This is the default for bwa mem.

  • What pre-processing of bam files is required?
    Bam files need to be sorted, indexed and ideally duplicate marked. If multiple libraries are present for a single sample these need to be merged in a single bam file with unique ReadGroup tags.

  • Usage/discussion mailing list?
    There is a delly discussion group delly-users.

  • Docker support?
    There is a dockerized delly available here.

  • Bioconda support?
    Delly is available via bioconda.


Tobias Rausch, Thomas Zichner, Andreas Schlattl, Adrian M. Stuetz, Vladimir Benes, Jan O. Korbel.
Delly: structural variant discovery by integrated paired-end and split-read analysis.
Bioinformatics 2012 28: i333-i339.


Delly is distributed under the GPLv3. Consult the accompanying LICENSE file for more details.