The Ctsk gene encodes a protein that is a lysosomal cysteine protease involved in bone remodeling and resorption. This protein is a member of the peptidase C1 protein family and is primarily expressed in osteoclasts. When Ctsk-iCre mice are crossed with mice containing loxP sites, the offspring can generate sequence deletions between loxP sites mediated by Cre recombinase in osteoclasts. The heterozygous Ctsk-iCre mice are viable and fertile. Reproductive leakage has been reported in this strain.