Hoxa2 null mice exhibit an 81% penetrance of cleft palate (Gendron-Maguire et al., 1993; Rijli et al., 1993; Barrow and Capecchi, 1999), which appears to result from increased cell proliferation where expression levels of both Msx1 and its known down-stream target Bmp4 are up-regulated during the early stages of palate development (Smith et al., 2009). Here, MSX1 is linked to cleft palate.