It has become apparent that tumor-infiltrating leukocytes, including neutrophils, macrophages, mast cells, eosinophils [4,5] as well as T cells with CD4+/CD25+ phenotype and suppressive function on helper and effector T cells, designated regulatory T cells (Tregs) [6,7], can cooperate in favouring, instead of antagonizing, tumor growth. This evidence concerns the gene CD4 and neoplasm.