Glycoprotein non-metastatic melanoma protein B (GPNMB), also known as osteoactivin given its role in osteopetrosis in rats (14), is widely expressed in various types of cells such as macrophages (15), dendritic cells (16), osteoblasts (17), melanocytes (18), neurons (19) and hepatocytes (20). Here, GPNMB is linked to osteopetrosis.