When they analyzed blood samples from patients with melanoma treated with the anti-GD2 CAR-T cells in the CARPETS phase I study, they found that, compared to the infused CAR-T cell product, harvested CAR-T cells had up-regulated PD-1 and PD-L1, while normal peripheral CD8+ T cells from the same patient had normal PD-1 expression. Here, PDCD1 is linked to melanoma.