Another molecule that plays a key role during bone resorption in lytic BM onset is cathepsin K. Cathepsin K is a papain-like cysteine protease involved in bone remodeling, highly expressed in osteoclasts and in activated reactive macrophages [109]; this protease affects cancer progression in several tumor types such as breast carcinoma and in prostate cancer [110]. The gene discussed is CTSK; the disease is breast carcinoma.