Tumor-infiltrating lymphocytes (TIL) [121,122], such as cytotoxic T cells, may predict better patient outcomes and responses to drugs (such as checkpoint blockade therapies); whereas, an increased number of regulatory T cells (Treg: CD4+FoxP3+CD25high) or myeloid-derived suppressor cells (MDSC) may correlate with lower survival rates in cancer and with lower clinical response rates to anti-CTLA-4 antibodies [123,124,125,126]. This evidence concerns the gene CD4 and cancer.