CD274 and neoplasm: Furthermore, because the tumor microenvironment is made up of heterogeneous cell populations, other biologic processes (potentially including but not limited to: secretion of growth factors and chemokines, production of blood vessels, myeloid-derived suppressor cell production of nitric oxide synthase and reactive oxygen species and infiltration of T-regulatory cells) may influence the effectiveness of PD-L1 as a biomarker among different tumor types [39].