HTRA1 and osteoporosis: SPP1 (secreted phosphoprotein 1), COL1A1 (collagen type I α 1 chain), NT5E (5′-nucleotidase ecto), HTRA1 (HtrA serine peptidase 1) and ANGPT1 (angiopoietin 1) and their signalling pathways involving osteoporosis in CD patients are identified.