English: Recruitment and maintenance of Treg cells in the tumor microenvironment. Multiple factors contribute to the increased frequency of Treg cells in tumors over normal tissues. Treg cells express specific chemokine receptors whose ligands are produced by various cells in the tumor microenvironment. Several mechanisms exist that contribute to the survival, stability, and proliferation of Treg cells in tumors, including conversion from CD4+ T cells, metabolic adaptation, and stability of FOXP3 expression. Neoantigen and self-antigen reactivity may also contribute to the prevalence of Treg cells in neoplastic tissues. Abbreviations: MHC II, major histocompatibility complex class II; NK, natural killer; TCR, T cell receptor; Teff, effector T cell; Treg, regulatory T.