CTSK and osteoporosis: Compared to wild-type mice, IRP2−/− mice showed obvious symptoms of osteoporosis, such as decreased bone mineral density and bone iron deficiency.115 The levels of bone formation markers in serum, including bone-gla-protein and type I collagen, were decreased, whereas those of bone resorption markers, such as tartrate-resistant acid phosphatase and cathepsin K, were significantly elevated.115 In the bone tissue of IRP2−/− mice, the expression of ferritin and FPN was reduced, while TFR1 expression was elevated, suggesting that iron disorders in bones may lead to osteoporosis.