A major complication that weakens the accounts of de novo pTreg induction after adoptive transfer of Teffs in tumor-bearing mice is that the CD4+CD25− Teffs subset purified in the majority of the conversion experiments of the pioneer articles, contains around 2% of CD4+CD25−Foxp3+ T-cells that exhibit suppressive functions (86) and can gain CD25 expression and expand after stimulation (87, 88). Here, CD4 is linked to neoplasm.