Small patella syndrome (SPS) is a rare syndrome that mainly affects the way certain bones are formed (developed). A person with SPS usually has very small kneecaps (hypoplastic patella) or may have no kneecaps at all (aplastic). The hip (pelvic) bones may be weak, and the bones of the feet may not have formed correctly. Occasionally, bones in the face may also be affected. Small patella syndrome is caused by a change (pathogenic variant) in the TBX4 gene. The syndrome is inherited in an autosomal dominant manner. Diagnosis of the syndrome is suspected based on features that can be seen in a physical exam and on imaging tests such as X-rays. Diagnosis of SPS can be confirmed with genetic testing.