CD38 and neoplasm: When stimulated, rTreg cells express Ki67, proliferate and convert into aTreg cells to exert their regulatory functions.18 The Ki67+ Treg subset identified in this study displayed significantly higher expression of FOXP3, CD38, HLA-DR, and CD39, resembling activated and proliferating Treg cells that possess high immune suppressive functions and may contribute to inducing a hypo-activated state of anti-tumor immunity.