PDCD1 and melanoma: However, by using flow cytometry and immunofluorescence, Tobias et al reported that a subpopulation of melanoma cells express PD-1, and that PD-1+ cancer cells are responsible for tumor initiation.127 In this study, PD-1 was primarily examined as a biomarker, enriched in ABCB5+ malignant melanoma-initiating cells.