CTSK and osteopetrosis: To date, at least 12 genes mutation including TCIRG1, CLCN7, LRP5, IGSF23, OSTM1, CAII, PLEKHM1, TNFSF11, TNFRSF11A, CTSK, IKBKG and ITGB3 have been reported to associated with human osteopetrosis.10, 11, 12, 13 Most of these mutations cause osteoclast‐rich osteosclerosis characterized by increasing the amount of osteoclasts but the activity of bone resorption is weakened or lost.11 However, the mutation of RANKL (TNFSF11) or its receptor RANK (TNFRSF11A) cause a kind of more rarely osteopetrosis characterized by lacking of mature osteoclasts.14, 15