PDCD1 and melanoma: Beyond this, in the latest study on better biomarkers of tumor-specific CD8+ T cells in the peripheral blood circulation, it was determined that three candidate biomarkers, including CD39, might compensate for the low sensitivity of PD-1 that served as a previous marker by single-cell RNA and TCR sequencing, at least in the blood of MC38 tumor-bearing mice and patients with melanoma (43).