Holoprosencephaly Type 1

**Semantic type:** Disease or Syndrome

**Definition:** The most severe form of holoprosencephaly in which there is a complete absence of midline forebrain division resulting in the presence of fused hemispheres and a single ventricle (alobar holoprosencephaly). It is mapped to chromosome 21q22.

**Synonyms:** - Holoprosencephaly 1