Mutational load/neoantigen-burden, basal level of tumor infiltrating T cells (TILs), differential expression of immune-checkpoint genes within the tumor tissue are important biomarkers that help predict the tumor’s predisposition toward immune-checkpoint inhibitors (ICIs) targeting IDO1, CTLA4, or PD-1 and increase the clinical success of immunotherapies. The gene discussed is CTLA4; the disease is neoplasm.