Since 1984, there has been a continual stream of reports describing the cloning of vertebrate genes containing homeo boxes characteristic of the Drosiphila Antp-like group. It is now clear that vertebrates, like insects, contain clustered ANTP-like genes; however, in contrast to Drosophila, mammals, aves and fish have more than one such HOX gene cluster. Furthermore, DNA sequence analyses suggest that genes within the insect HOM and mouse HOX-1, -2, -3 and -4 [HoxA, -B, -C and -D in new classification] clusters not only share a common ancestral Antp-like gene, but have evolved from an ancestral gene cluster. It is likely that common ancestor of insects and vertebrates possessed a cluster of 5 to 7 Antp-like genes. They have subsequently maintained their relative order along the chromosome despite different gene duplications in the lineages leading to insects and vertebrates. Thus, it is plausible that the ancestral cluster gave rise to 4 major HOX gene clusters in mammals, and to the homeotic genes in Drosophila. Two of mammalian HOX clusters  (HOX-3 and -4) may have arisen via partial duplication of the ancestral chromosomal region. Whilst HOX-1 and -2 are more extensive, and may have arisen via tandem duplication of some genes in the cluster, followed by a more recent chromosomal duplication. Due to this complexity, it is inaccurate to consider any Drosophila Antp-like gene to be a direct homologue of any single mammalian gene.

References:

Pendleton JW; Nagai BK; Murtha MT; Ruddle FH Expansion of the Hox gene family and the evolution of chordates. Proc Natl Acad Sci U S A 90: 6300-4 (1993)

Kappen C; Ruddle FH Evolution of a regulatory gene family: HOM/HOX genes. Curr Opin Genet Dev 3: 931-8 (1993)

Ruddle FH; Bartels JL; Bentley KL; Kappen C; Murtha MT; Pendleton JW Evolution of Hox genes. Annu Rev Genet 28: 423-42 (1994)