Hox genes are highly conserved master control genes that determine the identity of embryonic regions along the anteroposterior axis. They encode homeodomain transcription factors characterized by a 60 amino acid DNA binding domain (encoded by the 180 bp homeobox sequence element). The Hox genes are organized into gene complexes and are aligned along the chromosome in the same order as they are expressed along the anteroposterior axis. In Drosophila the HOX genes are divided into two complexes, the Antennapedia complex (ANT-C) and the bithorax complex (BX-C), separated by ~7.5 Mb on chromosomal arm 3R. (Adapted from FBrf0075367 and FBrf0194435).