Abstract The most common abnormality affecting the formation of the dentition is deviation from the usual number of the human permanent dentition (a total of 32 teeth in both jaws) or the deciduous dentition (20 total teeth in both jaws). When the number of teeth is less than the normal complement of dentition, the condition is known as hypodontia. Other anomalies seen in association with hypodontia include small tooth size (microdontia) and anomalies in tooth shape, most commonly tapering or “peg-shaped” teeth (McKeown et al., 2001; Brook et al., 2002). Frameshift, nonsense, and missense mutations in the human paired box gene 9 (PAX9) gene have been associated with hypodontia involving primarily posterior teeth. Severe hypodontia associated with a submicroscopic deletion involving loss of the entire gene in one family supports a model implicating haploinsuf)ciency of PAX9 as the primary cause for hypodontia involving molar teeth. Both primary and permanent dentitions are affected, with the effects being more prominent in the case of permanent teeth. PAX9 is a member of a family of transcription factors characterized by a common DNAbinding domain called the paired domain. It is highly homologous to PAX1, with which it shares an overlapping pattern of expression. PAX9 is a 342–amino acid protein encoded by a single gene mapping to human chromosome 14q21. Homozygous Pax9 mutant mice die shortly following birth. They lack a thymus, parathyroid glands, and ultimobranchial bodies. Mutant mice have a cleft secondary palate at birth, and tooth development is arrested at the bud stage, resulting in the absence of all teeth. Heterozygous mutants display no discernible defects.