Muscle segment homeobox 1 (MSX1) and Muscle segment homeobox 2 (MSX2) are members of the muscle segment homeobox gene family that encode transcription factors, playing important roles in the organogenesis and tissue–tissue interactions during vertebrate embryonic development [23], and mutations in MSX1 or MSX2 have been associated with impaired development of cranial neural crest-derived structures, oral clefts, and nonsyndromic oligodontia [23–27]. The gene discussed is MSX1; the disease is Oligodontia.