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. This evidence concerns the gene SIX3 and holoprosencephaly.