This was first recognized in 2001 when mutations in low-density lipoprotein 5 (LRP5), encoding a coreceptor for WNT ligands, were found to lead to low bone mass in the autosomal recessive osteoporosis pseudoglioma syndrome (OPPG, MIM 259770), characterized by early-onset severe osteoporosis and blindness (42, 43). The gene discussed is LRP5; the disease is osteoporosis.