HOX genes play a fundamental role in the development of the vertebrate central nervous system, heart, axial skeleton, limbs, gut, urogenital tract and external genitalia. The homeobox gene Hoxa-1 is transcriptionally regulated by retinoic acid (RA) and encodes a transcription factor, which has been shown to play important roles in cell differentiation and embryogenesis. Hoxa-1 is also expressed in cancers, such as mammary tumors, though it is not expressed in normal gland or in precancerous mammary tissues. At embryonic stages, Hoxa-2 is expressed in the mesenchyme and epithelial cells of palate, however its expression is restricted to the tips of the growing palatal shelves. Hoxa-2 protein is predominantly expressed in the nuclei of cells in the ventral mantle region of the developing embryo. In the developing and adult mouse spinal cord, Hoxa-2 protein may contribute to dorsal-ventral patterning and/or to the specification of neuronal phenotype. Hoxa-7 functions as a potent transcriptional repressor and its action as such requires several domains, including both activator and repressor regions. Hoxa-7 is expressed in the fetal liver, lung, skeletal muscle, kidney, pancreas and placenta.