The important role of genetics has been increasingly recognized in recent years with respect to the understanding of dental anomalies, such as tooth agenesis. The lack of any real insight into the cause of this condition has led us to use a human molecular genetics approach to identify the genes perturbing normal dental development. We are reporting a strategy that can be applied to investigate the underlying cause of human tooth agenesis. Starting with a single large family presenting a clearly recognizable and well-defined form of tooth agenesis, we have identified a defective gene that affects the formation of second premolars and third molars. With the use of "the family study" method, evidence is produced showing that other genetic defects also contribute to the wide range of phenotypic variability of tooth agenesis. Identification of genetic mutations in families with tooth agenesis or other dental anomalies will enable preclinical diagnosis and permit improved orthodontic treatment.