CST3 and osteoporosis: This suggests two possibilities: firstly, that osteoclast differentiation is increased in osteoporosis, and, in such conditions, cystatin C is not taken up by osteoclast progenitor cells, thus resulting in elevated serum cystatin C levels, and secondly that, in osteoporosis, the suppression of osteoclast differentiation may occur, thus resulting in a consequential increase in the expression of cystatin C. These mechanisms could explain why serum cystatin C increases in osteoporosis.