The Hox proteins play a role in patterns of embryonic development and cellular differentiation by regulating downstream target genes. The Hox11 gene, termed an orphan homeobox gene, as it is located outside of the four mammalian Hox clusters, is a DNA-binding nuclear transcription factor. The human Hox11 gene maps to chromosome 10q24 and has been implicated in the chromosomal translocation t(7;10)(q24;q11) that occurs in T-cell acute lymphoblastic leukemia (T-ALL). The t(7:10) translocation occurs between the Hox11 gene and the T-cell receptor (TCR) delta-chain gene and is a result of aberrant physiological recombinational events at the early stages of T-cell development. The Hox11 gene is normally expressed in the splanchnic anlage arising from the splanchnic mesoderm. Homozygous Hox11-deficient mice have no spleen, while all other splanchnic derivatives develop normally. Spleen development starts and proceeds normally in Hox11-deficient mice to a specific stage of embryogenesis, when the spleen anlage becomes fully absorbed.