COL1A2 and osteoporosis: Conventional osteoporosis-related markers are mostly protein biomarkers, such as bone formation-related serum total osteocalcin, bone alkaline phosphatase (ALP), type I procollagen N-terminal propeptide (PINP) and bone resorption-related type I collagen cross-linked C-terminal peptide, and serum type I collagen cross-linked N-terminal peptide (S-NTX) [32].