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 gr