Gene families consist of several copies of genes that encode very similar proteins. These likely arose due to gene duplications, and were altered by mutation over time to generate separate similar proteins. It is not a requirement that members of gene families have identical DNA or amino acid sequences. Both prokaryotes and eukaryotes have gene families.

A common example is the homeobox, or Hox, gene family, which codes for several proteins that are essential for developmental timing and orientation.