CYP27A1 and osteoporosis: The International Osteoporosis Foundation (IOF) and the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) suggest that the combination of a bone formation marker (serum procollagen type I N-terminal propeptide, PINP) and a bone resorption marker (serum C-terminal telopeptide of type I collagen, CTx) to be used as reference analytes for BTM measurements in clinical studies [4].