PDCD1 and melanoma: Hypothesizing that the IHC assay is not sensitive enough to detect levels of PD-L1 and PD-L2 that might indicate the efficacy of the checkpoint inhibitors, Morales-Betanzos et al. developed targeted MS assays to enable the measurements of PD-1, PD-L1, and PD-L2 in sections of human melanoma biopsies 67.