Dictionary.net

Anterior midline brain, cranial, and facial malformations resulting from the failure of the embryonic prosencephalon to undergo segmentation and cleavage. Alobar prosencephaly is the most severe form and features anophthalmia; cyclopia; severe MENTAL RETARDATION; CLEFT LIP; CLEFT PALATE; SEIZURES; and microcephaly. Semilobar holoprosencepaly is characterized by hypotelorism, microphthalmia, coloboma, nasal malformations, and variable degrees of mental retardation. Lobar holoprosencephaly is associated with mild (or absent) facial malformations and intellectual abilities that range from mild mental retardation to normal. Holoprosencephlay is associated with CHROMOSOME ABNORMALITIES.