The GCG sequence format is part of the GCG Wisconsin Sequence Analysis Package, developed by the Genetics Computer Group A sequence file in GCG format represents exactly one genetic string. It begins with annotation lines, and the start of the sequence is marked by a line containing the sequence's ID, length, and a checksum, followed by two dots (".."). An example file in GCG format is the human mRNA represented below by its pre-transcribed DNA string:

ID   AB000263 standard; RNA; PRI; 368 BP.
AC   AB000263;
DE   Homo sapiens mRNA for prepro cortistatin like peptide, complete cds.
SQ   Sequence 368 BP;
AB000263  Length: 368  Check: 4514  ..
       1  acaagatgcc attgtccccc ggcctcctgc tgctgctgct ctccggggcc acggccaccg
      61  ctgccctgcc cctggagggt ggccccaccg gccgagacag cgagcatatg caggaagcgg
     121  caggaataag gaaaagcagc ctcctgactt tcctcgcttg gtggtttgag tggacctccc
     181  aggccagtgc cgggcccctc ataggagagg aagctcggga ggtggccagg cggcaggaag
     241  gcgcaccccc ccagcaatcc gcgcgccggg acagaatgcc ctgcaggaac ttcttctgga
     301  agaccttctc ctcctgcaaa taaaacctca cccatgaatg ctcacgcaag tttaattaca
     361  gacctgaa