SIX3 and holoprosencephaly: Loss of Six3 function produces essentially forebrain defects rostral to the prethalamus (diencephalon), with the absence of the telencephalon, hypothalamus, eyes, and pituitary gland, as well as the olfactory placodes (Carl et al., 2002; Lagutin et al., 2003), a phenotype similar to severe holoprosencephaly phenotypes in humans (Cohen, 2006; Dubourg et al., 2007).