For example, PD-1 (encoded by PDCD1) and PD-L1 (encoded by CD274), a well-defined immunosuppressive axis in the TME [36–38], were found to be correlated with SPHK1 expression in most cancer types, including skin cutaneous melanoma (SKCM; Fig. 1f, g). Here, PDCD1 is linked to cutaneous melanoma.