PARP1 and cancer: Examples include phosphorylated PD-L1 by glycogen synthase kinase 3β (GSK3β), which can be further degraded by ubiquitin-proteasome system and Olaparib, a PARP1 inhibitor that maintains and upregulates the expression level of PD-L1 in cancers by suppressing the activity of GSK3β, thereby inhibiting the antitumor immune response and facilitating the tumor immune escape (142).