Homozygous mutations of the tartrate-resistant acid phosphatase gene (ACP5), encoding for the protein TRAP, cause the immune-osseous disease, SPENCD [80, 81], which is characterized by platispondily, enchondromatosis, brain calcifications, spasticity and autoimmunity including SLE with malar rash, lupus nephritis, antiphospholipid syndrome and anti-dsDNA antibodies. Here, ACP5 is linked to lupus nephritis.