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Circulating osteocalcin, a marker of bone formation, is under strong genetic influence, and this effect is related to the genetic influence on bone density. To examine genetic influences on bone turnover further, other markers of bone formation (serum carboxyterminal propeptide of type I procollagen, PICP), bone resorption (serum pyridinoline cross-linked carboxyterminal telopeptide of type I collagen, ICTP), and nonosseous connective tissue synthesis (serum aminoterminal propeptide of type III procollagen, PIIINP) were studied in 82 female twin pairs: 42 monozygotic (MZ) and 40 dizygotic (DZ) twin pairs (mean age, MZ; 48.4 yr; DZ; 45.6 yr). The intraclass correlation coefficients of MZ twin pairs, rMZ, for serum PICP (0.78) and serum ICTP (0.68) were significantly greater than the corresponding rDZ (0.31 and 0.36, respectively), but a genetic effect on serum PIIINP was not demonstrable. Within DZ twin pair …