PDCD1 and Marinesco-Sjogren syndrome: While MSI-high (MSI-H, referring to high levels of gene instability) disease benefits markedly from PD-1-based immunotherapy (drugs that inhibit the PD-1 protein on immune cells), TP53-mutated MSS tumors rarely receive immune checkpoint inhibitors (ICIs, drugs that help immune cells attack cancer) outside of trials and often only in later lines of therapy.