Diprosopia is a rare form of conjoined twinning where typically only the face is duplicated. In the few cases reported, the facial structures are duplicated with multiple abnormalities such as cleft lip and palate, holoprosencephaly (where the front part of the brain does not develop), or part or most of the brain is missing (anencephaly). In some cases the trunk of the body can be normal, however, in most cases, the body may have additional abnormalities of the spine, chest cavity, respiratory tract, gastrointestinal tract, and heart defects. Though this is considered a rare form of conjoined twinning, it is not normally caused by two embryos that fuse together or don't separate completely. It is thought to be due to errors in the Sonic Hedgehog gene that normally directs the pattern of craniofacial (head and face) development. Due to the rarity of this condition, there is much to be learned.