1) 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 characterised 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. Holoprosencephaly is associated with chromosome abnormalities.

2) Holoprosencephaly is a developmental disorder that results when the forebrain of the embryo fails to divide and form the right and left halves of the brain. The disorder produces a single-lobed brain structure and severe skull and facial abnormalities. Often the deformities cause babies to die before birth. In mild cases, babies are born with near-normal brain development and facial abnormalities involving cleft lip or cleft palate.