Loss of a single copy of Shh, or mutations that lead to reduced expression of Shh in prechordal mesoderm, result in holoprosencephaly (Belloni et al., 1996; Geng et al., 2008; Leung et al., 2010; Roessler et al., 1996; Roessler and Muenke, 2010). The gene discussed is SHH; the disease is holoprosencephaly.