PAX9 and Hypodontia: Interestingly, most of the mutations in the MSX1 gene—which, so far, have been associated with hypodontia in humans—are located in this part of the gene that codes for a highly conservative homeodomain which is responsible for the ability of the transcription factor to bind to specific regions of DNA in target regulatory genes and interact with other regulatory proteins, such as, for example, PAX9.