MAP2K1 and Smith-Lemli-Opitz syndrome: MAP2K1 is a key signaling molecule in MAPK and Ras signaling pathways, which are known to play important roles in precursor proliferation and differentiation (Bonni et al., 1999; Ménard et al., 2002; Yang et al., 2013), consistent with the phenotypes that were observed in Dhcr7-/- mice and SLOS patient-derived hNPCs in vitro.