GPNMB is a type I transmembrane glycoprotein which shows homology to the pmel17 precursor, a melanocyte-specific protein. GPNMB has been reported to be expressed in various cell types, including: melanocytes, osteoclasts, osteoblasts, dendritic cells, and it is overexpressed in various cancer types. GPNMB was originally identified as a gene that was expressed in poorly metastatic human melanoma cell lines and xenografts and not expressed in highly metastatic cell lines. However, several recent studies have identified high GPNMB expression in aggressive melanoma, glioma, and breast cancer specimens.