However, other subtypes of tumor-infiltrating CD4+ T cells may play a pro-tumorigenic role in the tumor microenvironments via the secretion of inflammatory or regulatory cytokines, such as interleukin (IL)-6, IL-10, IL-17, IL-21, and transforming growth factor (TGF)-β, as the abundance of such CD4+ T cells has been associated with a poor clinical outcome of various types of cancer [1–4]. This evidence concerns the gene IL21 and cancer.