Inpatients with haemophilia, increased osteoclastic activity (increase in tartrate-resistant acid phosphatase isoform-5b-TRACP-5b, N-terminal cross-linking telopeptide of collagen type I-NTX, and C-terminal cross-linking telopeptide of collagen type I-CTX), but without increased bone formation (decrease in osteocalcin) were shown. The gene discussed is ACP5; the disease is hemophilia.