To date, six genes have been associated with MED, including five genes that cause autosomal dominant MED (AD-MED; COMP, COL9A1, COL9A2, COL9A3 and MATN3) and one gene that causes autosomal recessive MED (rMED; SLC26A2) [2–4]. The gene discussed is COL9A3; the disease is Alzheimer disease.