I. To evaluate ORR, PFS, and OS based on expression of programmed cell death 1 ligand 1 (PD-L1), programmed cell death 1 (PD-1), peritumoral cluster of differentiation (CD)8+ tumor infiltrating lymphocytes (TILs), peritumoral CD4+ TILs, and regulatory T cells as analyzed from tumor biopsies in previously treated patients with metastatic SCCA of the anal canal when treated with nivolumab.