The Hox proteins play a role in development and cellular differentiation by regulating downstream target genes. Specifically, the Hox proteins direct DNA-protein and protein-protein interactions that assist in determining the morphologic features associated with the anterior-posterior body axis. HoxA13 and HoxD13 also bind to other BMP and TGF-beta/Activin-regulated Smad proteins including Smad1 and Smad2, but not Smad4. In humans and mice, loss of HOXA13 function causes defects in the growth and patterning of the digits and interdigital tissues. Analysis of HoxA13 expression reveals a pattern of localization overlapping with sites of reduced Bmp2 and Bmp7 expression in HoxA13 mutant limbs. HoxA13 regulates Bmp2 and Bmp7 expression, providing a link between HoxA13, its target genes and the specific developmental processes affected by loss of HoxA13 function.