Regulatory T (Treg) cells expressing the transcription factor forkhead box P3 (Foxp3), most of which are CD4+ T cells that express CD25 (the interleukin-2 (IL-2) receptor α-chain), are indispensable for the maintenance of dominant self-tolerance and immune homeostasis, but also suppress antitumour immune responses and favour tumour progression. Here, CD4 is linked to neoplasm.