In peptide-based cancer vaccines, usually 20–30 amino acids are used to make a wide range of peptides for activating the immune system of patients, enabling them to recognize and kill the tumor cells by enhancing the T cell-mediated immune responses specific to a particular tumor, i.e., CD8+ and CD4+ T cells via the MHC class I and II molecules, respectively [106,107]. The gene discussed is CD4; the disease is cancer.