...is the title of an article by the US Department of Health and Human Services, National Institue of Environmental Health Sciences released in January 26, 2007.
According to a research, a certain type of vitamin B reduced the risk of isolated cleft lip with or without palate by 1/3 (33%).
http://www.niehs.nih.gov/oc/news/01262007-0702.htm
Though my wife and I gravitate toward genetics as the main cause, I found this article very interesting and I am sure you will too.
Gregory (father 10 yoB born with PRS)