Hereditary multiple exostoses  Hereditary multiple osteochondromas (HMO), also known as hereditary multiple exostoses, is a disorder characterized by the development of multiple benign osteocartilaginous masses (exostoses) in relation to the ends of long bones of the lower limbs such as the femurs and tibias and of the upper limbs such as the humeri and forearm bones. They are also known as osteochondromas. Additional sites of occurrence include on flat bones such as the pelvic bone and scapula. The distribution and number of these exostoses show a wide diversity among affected individuals. Exostoses usually present during childhood. The vast majority of affected individuals become clinically manifest by the time they reach adolescence. A small percentage of affected individuals are at risk for development of malignant transformation namely sarcomas. The incidence of hereditary multiple exostoses is around 1 in 50,000 individuals. Hereditary multiple osteochondromas is the preferred term used by the World Health Organization. A noticeable lump in relation to an extremity may be the first presenting symptom. Multiple deformities can arise, namely coronal plane deformities around the knees, ankles, shoulders, elbows, and wrists. For example, genu valgum (knock knees), ankle valgus, ulnar bowing and shortening, and radial head subluxation are encountered. The majority of affected individuals have clinically manifest osteochondromas around the knee. Forearm involvement in HMO is considerable. Furthermore, short stature may occur and is generally disproportionate. Such manifestations usually result from disruption of physeal growth especially that osteochondromas typically arise at the metaphyseal ends of long bones in close proximity to the physis. Intra-articular osteochondromas of the hip can induce limitation of range of motion, joint pain and acetabular dysplasia. Likewise joint pain at other locations and neurovascular compression can occur. Furthermore, functional disability in regard to activities of daily living can be a presenting feature. Spinal deformity pain or neurological compromise should arouse suspicion of involvement of the vertebrae. Some parents of children with HME have observed autism-like social problems in their children. To explore those observations more deeply, a 2012 study by the Sanford-Burnham Medical Research Institute used a mouse model of HME to observe cognitive function. The findings indicated that the mutant mice endorsed three autistic characteristics: social impairment, impairments in ultrasonic vocalization, and repetitive behavior.