The homeotic Hox proteins are sequence-specific transcription factor which are part of a developmental regulatory system that provides cells with specific positional identities on the anterior-posterior (A-P) axis .The hox proteins contain an `homeobox' domain. In Drosophila and other insects there are eight different Hox genes that are encoded in two gene complexes: ANT-C and BX-C; while in vertebrates there are 38 genes organized in four complexes. Recently [2,3] a new nomenclature system has been proposed for Hox genes. The four gene complexes are assigned the letters A to D. Based on sequence similarities the genes can be sorted into 13 `paralog' groups. The order of the paralogs along the chromosomes are conserved in the four complexes. The gene name is obtained by concatenating the gene complex letter designation with the group number; examples: HOXA1, HOXB4, HOXC8, HOXD13, etc. In several cases a representative of a paralog group is absent from a complex, in such cases that gene number is omitted. For example: HOXA8, HOXC1 andHOXD2 do not exist.
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