FOXP3 and lymphopenia: The manifestation of some mild disease symptoms (moderate growth delay and mild exfoliative dermatitis) can probably be attributed to an initial lag phase after tTreg cell-depleted CD4+ T cell transfer, which is required for seeding and proliferative expansion of pre-formed pTreg cells, and the lymphopenia-driven de novo generation of Foxp3+ pTreg cells (20).