CD4 and neoplasm: Furthermore, mechanisms of Treg modulation by radiation are elucidated by Oweida and his colleagues in an orthotopic mouse model of head and neck cancer (HNC).150 They demonstrated that signal transducer and activator of transcription 3 (STAT3), as a key molecule to regulate FOXP3,151,152 promotes the conversion induced by radiation from CD4+ T cells to Treg cells and STAT3 inhibition in combination with radiation changes tumor microenvironment dramatically in the aspect of decreasing Treg cells, MDSCs and M2 macrophages along with enhancing effector T cells and M1 macrophages.