GAS1 and holoprosencephaly: Significantly, loss-of-function associated with GAS1 and CDON have both been associated with HPE in humans (Bae et al., 2011; Ribeiro et al., 2010) and mice (Allen et al., 2007; Cole and Krauss, 2003; Martinelli and Fan, 2007; Seppala et al., 2007; Tenzen et al., 2006; Zhang et al., 2011; Zhang et al., 2006) (supplementary material Table S1); whilst Boc mutant mice lack HPE but do have misguided commissural axon guidance, cerebellum reduction and reduced ipsilateral retinal ganglion cells (Izzi et al., 2011; Okada et al., 2006; Sánchez-Arrones et al., 2013).