Chapter 1. FASTA Format

The most common sequence format you'll encounter is FASTA. This format is quite simple. The first line of a sequence entry consists of ">", followed by an identifier, which contains no whitespace. This can be followed by whitespace and a comment or description. This first line is referred to as the comment or description line. One or more sequence data lines may follow. The length of the sequence data lines may not be constant. Common line lengths are 60, 70, 72, and 80. For details, see Section 1.3 at the end of this chapter. Example 1-1 contains a sample FASTA entry.

Example 1-1. Sample FASTA entry

>gi|29848|emb|X61622.1|HSCDK2MR H.sapiens CDK2 mRNA
ATGGAGAACTTCCAAAAGGTGGAAAAGATCGGAGAGGGCACGTACGGAGTTGTGTACAAAGCCAGAAACA
AGTTGACGGGAGAGGTGGTGGCGCTTAAGAAAATCCGCCTGGACACTGAGACTGAGGGTGTGCCCAGTAC
TGCCATCCGAGAGATCTCTCTGCTTAAGGAGCTTAACCATCCTAATATTGTCAAGCTGCTGGATGTCATT
CACACAGAAAATAAACTCTACCTGGTTTTTGAATTTCTGCACCAAGATCTCAAGAAATTCATGGATGCCT
CTGCTCTCACTGGCATTCCTCTTCCCCTCATCAAGAGCTATCTGTTCCAGCTGCTCCAGGGCCTAGCTTT
CTGCCATTCTCATCGGGTCCTCCACCGAGACCTTAAACCTCAGAATCTGCTTATTAACACAGAGGGGGCC
ATCAAGCTAGCAGACTTTGGACTAGCCAGAGCTTTTGGAGTCCCTGTTCGTACTTACACCCATGAGGTGG
TGACCCTGTGGTACCGAGCTCCTGAAATCCTCCTGGGCTCGAAATATTATTCCACAGCTGTGGACATCTG
GAGCCTGGGCTGCATCTTTGCTGAGATGGTGACTCGCCGGGCCCTGTTCCCTGGAGATTCTGAGATTGAC
CAGCTCTTCCGGATCTTTCGGACTCTGGGGACCCCAGATGAGGTGGTGTGGCCAGGAGTTACTTCTATGC
CTGATTACAAGCCAAGTTTCCCCAAGTGGGCCCGGCAAGATTTTAGTAAAGTTGTACCTCCCCTGGATGA
AGATGGACGGAGCTTGTTATCGCAAATGCTGCACTACGACCCTAACAAGCGGATTTCGGCCAAGGCAGCC
CTGGCTCACCCTTTCTTCCAGGATGTGACCAAGCCAGTACCCCATCTTCGACTCTGATAGCCTTCTTGAA
GCCCCCGACCCTAATCGGCTCACCCTCTCCTCCAGTGTGGGCTTGACCAGCTTGGCCTTGGGCTATTTGG
ACTCAGGTGGGCCCTCTGAACTTGCCTTAAACACTCACCTTCTAGTCTTAACCAGCCAACTCTGGGAATA
CAGGGGTGAAAGGGGGGAACCAGTGAAAATGAAAGGAAGTTTCAGTATTAGATGCACTTAAGTTAGCCTC
CACCACCCTTTCCCCCTTCTCTTAGTTATTGCTGAAGAGGGTTGGTATAAAAATAATTTTAAAAAAGCCT
TCCTACACGTTAGATTTGCCGTACCAATCTCTGAATGCCCCATAATTATTATTTCCAGTGTTTGGGATGA
CCAGGATCCCAAGCCTCCTGCTGCCACAATGTTTATAAAGGCCAAATGATAGCGGGGGCTAAGTTGGTGC
TTTTGAGAATTAAGTAAAACAAAACCACTGGGAGGAGTCTATTTTAAAGAATTCGGTTAAAAAATAGATC
CAATCAGTTTATACCCTAGTTAGTGTTTTCCTCACCTAATAGGCTGGGAGACTGAAGACTCAGCCCGGGT
GGGGGT

Many organizations have specific syntax for the description line and have written their own code for parsing and writing FASTA files. Most open source tools expect only the identifier, and treat the rest of the line as a single description string.

A FASTA file may contain more than one sequence entry. The entries are merely concatentated, with the ">" prefixed lines indicating the start of a new sequence entry.