CYP7B1 and holoprosencephaly: For example, CDH23 is mutated in Usher syndrome, characterised by profound deafness38, CYP7B1 is mutated in a rare form of spastic paraplegia39, SIX3 is mutated in holoprosencephaly resulting in major mental retardation40, and mutations in FOXP2 are associated with speech and language disorder 141.