IRF1 and squamous cell carcinoma: Depending on stimulus and cell type, the expression of PD-L1 was found to connect with various signaling molecules: MAPKs [30], STAT family [31,32], IRF-1 [33], PIK3 [34] and etc. Studies have shown that PD-L1 is more highly expressed in squamous carcinoma, both at the protein level and at the mRNA level.