FGF8 and holoprosencephaly: Furthermore, FGF8 and SIX3 have been found to be mutated in holoprosencephaly (HPE) (Dubourg et al., 2016; McCabe et al., 2011; Wallis et al., 1999), a common developmental defect in midline patterning of the forebrain and/or midface (Dubourg et al., 2007; Marcorelles and Laquerriere, 2010).