Treg cells are known to differentiate into IFN-γ- or IL-17-producing Treg cells, or to transdifferentiate into Foxp3-negative Th1, Th17 or Tfh cells, referred to as exTreg cells, under lymphopenic or inflammatory conditions in mice [26,29], and in patients with cancer, autoimmune disease or infections [17,27,28,45,46,47]. Here, IL17A is linked to cancer.