MED is genetically heterogeneous and can be caused by mutations in genes encoding the extracellular matrix (ECM) proteins matrilin-3, type IX collagen and cartilage oligomeric matrix protein (COMP) (Briggs and Chapman, 2002; Jackson et al., 2012); PSACH results exclusively from COMP mutations (Jackson et al., 2012) and is more severe than MED, but both phenotypes comprise a disease spectrum with symptoms that can include joint pain and stiffness, lower-limb deformities and early onset osteoarthritis (Briggs and Chapman, 2002). The gene discussed is COMP; the disease is multiple epiphyseal dysplasia.