Osteochondromas are non-cancerous (‘benign’) bone tumours that look like small, hard lumps on the outside of bones. These may be painful, and in rare cases can turn into cancer. Individuals with hereditary multiple osteochondromas are at increased chance of developing these lumps due to disease-causing (‘pathogenic’) changes in EXT1 or EXT2, genes which have a role in controlling normal bone growth. Additional features of the condition include bone deformities, short stature, or limitations in joint movement. The condition is inherited in an autosomal dominant pattern and as high penetrance by adolescence. The severity of the condition, including the number, size, and location of these lumps can vary significantly between individuals (‘variable expressivity’).