Therefore, due to the critical role of STAT5 signalling in Treg development, the significantly higher levels of basal pSTAT5 in the aTreg subset compared to FOXP3− Tcon cells detected in treated patients with advanced CLL may be responsible for the perturbed homeostasis between the aTreg and Tcon subsets of CD4+ T cells. Here, FOXP3 is linked to B-cell chronic lymphocytic leukemia.