cases. Cause TDO is caused by autosomal dominant mutations in the DLX gene family. Known mutations include a 4-basepair deletion in exon 3 of DLX3 on chromosome 17q21, causing a frameshift mutation. Pathogenesis DLX3 plays numerous significant roles during osseous, connective tissue, and dermal cell differentiation. It is expressed in the placenta and is important for embryonic development of hard bone tissue in the teeth, skull, and long bones such as in the arms and legs. During normal tooth development, DLX3 is predominantly expressed in the inner tooth enamel epithelium. In TDO cases, DLX3 is expressed in the outer enamel epithelium and leads to the abnormally thin enamel observed in the disease, which leads to tooth attrition and is most often the cause of dental abscess seen in TDO persons. DLX3 in TDO is also responsible for upper cranial thickness, calvaria, osteosclerosis of the long bones, long narrow head (dolichocephaly), abnormally thin brittle nails, and premature closing of fibrous joints. Consequently, 95% of people with TDO that are 16 years old or younger show skeletal abnormalities before full maturation takes place. Lack of mastoid pneumatization by mastoid cells occurs in 82% of the cases and is rarely prevalent outside of TDO diagnosis. Mastoid pneumatization occurs at about 6 months of ages and acts to minimize pressure fluctuations in the Eustachian tubes of the ear. The mastoid lies posterior to the lower jawbone (mandible) and distal to the ear. The Eustachian tube connects the middle ear to the back of the nose, and acts to create a specific pressure in the ear canal that causes vibrations to the eardrum; if adequate pressure is not attained, muffled, dull hearing results. In addition to the mastoid pneumatization assisting the Eustachian tubes for normal hearing, lack of mastoid pneumatization causes inflammation of the ear, general irritation, and does not allow enough air in to assist with mucus flowing out. It is not completely understood why gene mutations occur, but it is known that genetic mutations that cause disease are acquired from either or both parents at fertilization. Diagnosis TDO is diagnosed by radiographic imaging, physical characteristics of the disease, and genetic testing. Radiographs such as cephalometric analysis or panoramic radiograph are used to detect skeletal abnormalities in TDO cases; these radiographs along with the phenotypic effects of the disease are often enough evidence for proper diagnosis. In TDO, radiologic imaging almost always shows evidence