CYP27A1 and pycnodysostosis: Consistently, the urine levels of cross- linked N- and C-telopeptides of type I collagen (NTX and CTX), two bone resorption markers reflecting the degradation of type I collagen, were both decreased in the pycnodysostosis (Pycno) patients (Nishi et al., 1999), which suggests the low bone resorption activity in pycnodysostosis.