Holoprosencephaly is a common human brain malformation; there are many different known causes.  Sonic Hedgehog was the first gene in which mutations were found to cause holoprosencephaly .  Some individuals with Sonic Hedgehog mutations have very severe brain and facial malformations, and do not survive to be born.  However, some people have only subtle facial differences.  We examined almost all known individuals (almost 400 total) with Sonic Hedgehog mutations, and describe the clinical manifestations.  We found that certain types of Sonic Hedgehog mutations tend to cause severe disease, while other types of mutations are more likely to cause milder findings. (By Maximilian Muenke, M.D., http://jmg.bmj.com/content/49/7/473 )