In the early 1960s scientists deciphered the genetic code of life, determining the rules by which information in DNA molecules is translated into proteins, the working parts of living cells. They identified three-letter units in DNA sequences, known as codons, that specify each of the 20 amino acids that make up proteins. Occasionally, single-letter misspellings in the genetic code, known as point mutations, occur. Point mutations that alter the resulting protein sequences are called nonsynonymous mutations, while those that do not alter protein sequences are called silent or synonymous mutations. Between one-quarter and one-third of point mutations in protein-coding DNA sequences are synonymous. Ever since the genetic code was cracked, those mutations have generally been assumed to be neutral, or nearly so.