Studies have evaluated the expression of PD-1/PD-L1 on circulating exosomes in melanoma patients and found that higher levels of PD-1/PD-L1+ exosomes were significantly correlated with poorer progression-free survival (PFS) and overall survival (OS), which identified circulating PD-1+ exosomes as the driving factor of anti-PD-1 resistance and emphasized that analysing a single exosome population through liquid biopsy was a promising tool for immunotherapy stratification of melanoma patients [214]. Here, PDCD1 is linked to melanoma.