is an abnormality of brain development that also affects the head and face. Normally, the brain divides into two halves (hemispheres) during early development. Holoprosencephaly occurs when the brain fails to divide properly into the right and left hemispheres. There are four types of holoprosencephaly according to the degree of brain division. From most to least severe, the types are known as alobar, semi-lobar, lobar, and middle interhemispheric variant (MIHV). The most severely affected individuals have one central eye (cyclopia) and a tubular nasal structure (proboscis) located above the eye. In the less severe forms, the brain is partially divided and the eyes are usually set close together (hypotelorism). Holoprosencephaly can be caused by mutations in a single gene, chromosome abnormalities, or substances that cause birth defects (teratogens). Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.