2BIT File

What are 2BIT files and how to open them

Are you having problems opening a 2BIT file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your 2BIT files.

What is a 2BIT file?

A .2BIT file is a 2BIT DNA Sequences (LE) file.

The 2BIT DNA Sequences format is a compact binary file used by the UCSC Genome Browser to store multiple genomic DNA sequences and enable fast, random access. It is created from FASTA files by UCSC's command-line utility called faToTwoBit. This format is ideal when you have large genome assemblies and want efficient storage and quick retrieval of sequence segments without loading entire genomes into memory.

Files in this format are read by UCSC tools such as twoBitToFa (which extracts sequences in FASTA form), twoBitInfo (which lists sequences and their sizes), and by genome browsers or services that need to fetch specific regions of a genome. It includes support for "masked" sequence regions (lower-case letters) and for representing unknown bases (Ns).

Internally, a 2bit file begins with a 16-byte header containing a signature (magic number), version (currently zero), the total count of sequences in the file, and a reserved field. Following the header is an index listing each sequence by name and file offset. Each sequence block then encodes: dnaSize (number of bases), lists of N‐blocks (unknown bases) and mask blocks (lower-case sequence), and finally the DNA itself packed at 2 bits per base using the code T=00, C=01, A=10, G=11. The file signature is 0x1A412743 when read in the native little-endian order, or byte-swapped (0x4327411A) if endianness differs.

How to open 2BIT files

Important: Different programs may use files with the 2BIT file extension for different purposes, so unless you are sure which format your 2BIT file is, you may need to try a few different programs.

We have not yet verified any programs for Windows that work with this specific file format. If you know one, please use the 'Suggest a program' link below. Thanks!

Last updated: August 16, 2026

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