N-terminal propeptide of type I procollagen (PINP) and C-telopeptide of type I collagen (CTX-I) are markers of bone formation and resorption, respectively, that the International Osteoporosis Foundation and the International Federation of Clinical Chemistry recommend for clinical use.
The conclusion that most variants of OI are caused by mutations in the structural genes for type I procollagen has broad implications for other diseases that affect connective tissue, diseases such as chondrodystrophies, osteoarthritis, and osteoporosis.
One, a cysteine substitution in alpha 1(I) collagen, causes a mild Sillence type I disease, the other, a four base deletion in the C terminal extension of alpha 2(I) collagen, causes progressive Sillence type III disease in the homozygously affected patient and mild premature osteoporosis in his clinically symptomless parents.