In melanoma and other tumor types, clinical trials have focused on immunohistochemical (IHC) staining of programmed death ligand-1 (PD-L1) as a potential predictive biomarker for response.[2] PD-L1 is one of several ligands to PD-1, and can be expressed on either tumor cells or stromal cells.[12] While PD-L1 expression in pre-treatment tumors is generally associated with higher response rates [13], the limited predictive power of this biomarker has discouraged its development as a companion diagnostic in melanoma. The gene discussed is CD274; the disease is neoplasm.