The presence of an altered Hox-2.4 gene in the WEHI3B murine myeloid leukemia suggests that homeobox genes may contribute to neoplasia. A survey of 31 leukemia cell lines of the myeloid, lymphoid and erythroid lineages revealed that Hox-2.4 was expressed only in WEHI3B and the pre-B lymphoid line 70Z/3, in which no DNA rearrangement was observed. To clarify the WEHI3B alteration and normal Hox-2.4 structure, we have sequenced near full length cDNA clones from WEHI3B and 70Z/3, and the 5' portion of the normal Hox-2.4 gene. A WEHI3B cDNA clone demonstrates that an intracisternal A-particle (IAP) provirus has inserted within the first exon of the gene and generated a Hox-2.4 mRNA with a 5' sequence derived from the IAP long terminal repeat. A remarkable degree of similarity found between the amino acid sequences of Hox-2.4 and Hox-3.1, which reside on different chromosomes, supports the notion that an ancient homeobox gene cluster has been duplicated and dispersed early in vertebrate evolution.