CD274 and melanoma: As one of the most important player in the immune checkpoint system, PD-L1 has been reported to be expressed in 7 types of cancers, including urothelial [9], lung [10], thyroid [11], cervical [12], skin [13], stomach [14] and melanoma [15], and membranous PD-L1 functions as a co-inhibitory factor in immune regulation [16].