Our cells read the sequence of chemical letters in DNA to make proteins, and the DNA corresponding to one protein is called a gene. (en.hdbuzz.net)
HapMap is a directory of «single nucleotide polymorphisms,» or SNPs, places in the genome where differences between individuals (in the form of single chemical letters) appear in the DNA code. (discovermagazine.com)
Yet, scientists don't know why DNA spells out genes with four chemical letters instead of two or six. (sciencemag.org)