Ulnar-mammary syndrome (UMS) is a rare genetic syndrome that affects the development of several parts of the body. Features of UMS can be mild to severe and can vary significantly from person to person, even within the same family. The main features of UMS include upper limb defects (including abnormal or incomplete development of the fingers and forearm), underdevelopment of the mammary and apocrine glands (leading to absent breast development and the inability to produce breast milk), and various genital abnormalities. Other signs and symptoms may include hormonal deficiencies, delayed puberty (particularly in males), dental problems, short stature, and obesity. People with UMS may have distinct facial features, including a wide face tapering to a prominent chin, and a broad nose. UMS is caused by genetic changes in the TBX3 gene and inheritance is autosomal dominant. However, not all people who have or inherit a genetic change will have features of UMS. This phenomenon is called incomplete penetrance.