CRAM 3.1: Advances in the CRAM Format

MotivationCRAM has established itself as a high compression alternative to the BAM file format for DNA sequencing data. We describe updates to further improve this on modern sequencing instruments. ResultsWith Illumina data CRAM 3.1 is 7 to 15% smaller than the equivalent CRAM 3.0 file, and 50 to 70% smaller than the corresponding BAM file. Long-read technology shows more modest compression due to the presence of high-entropy signals. AvailabilityThe CRAM 3.0 specification is freely available from https://samtools.github.io/hts-specs/CRAMv3.pdf. The CRAM 3.1 improvements are available from https://github.com/samtools/hts-specs/pull/433, with OpenSource implementations in HTSlib and HTScodecs. Contactjkb@sanger.ac.uk Supplementary informationSupplementary data are available online