n. a unit of the genetic code consisting of three consecutive bases in a DNA or messenger RNA sequence. Most codons specify a particular amino acid in protein synthesis, although some act as “start” or “stop” signals. Codons on a gene are numbered, and genetic mutations are described as occurring at particular codons or locations on the gene. Thus, a family might be described as having a mutation at codon 630 on exon 11 of chromosome 10.