CD274 and melanoma: Consequently, although the JAK/STAT/IRF1 pathway is critical for the IFNγ-mediated induction of HLA-ABC, HLA-DR, and the two PD-1 ligands (14, 23), the low constitutive expression of PD-L1 suggests that this pathway has low baseline activity in melanoma and that the constitutive expression of HLA-ABC, HLA-DR, PD-L2, and NGFR may be regulated via alternate pathways or downstream elements.