Holoprosencephaly (HPE) is a birth defect in which the brain fails to form properly. The most severe form is Alobar Holoprosencephaly, in which the brain does not divide into two hemispheres. These children often have significant facial malformations as well. In Semilobar Holoprosencephaly, the brain is divided in the back but not in the front; in Lobar Holoprosencephaly, the hemispheres are divided but are fused in some areas; and in cases with Middle Interhemispheric Variant, the brain is not well divided in the middle. Mental retardation, epilepsy, microcephaly, hydrocephaly, and malformations in other organs are all associated with holoprosencephaly.