We already know several genes which, when defective, cause congenitally missing teeth. The known gene defects include mostly those that cause a multi-organ syndrome (genes EDA, EDAR, EDARADD, IKKgamma, p63, IRF6, PITX2, TFAPB2, SHH, OFD1). Only two genes are known so far where defects cause isolated tooth agenesis. Dominant loss-of function mutations in MSX1 and PAX9 cause oligodontia. Identication of gene defects that cause isolated hypodontia, the most common type of congenitally missing teeth, has been much more difficult.