FOXP3 and chronic myelogenous leukemia, BCR-ABL1 positive: To determine the cellular origin of Tregs in CML, we stained for Helios and neuropilin-1, two markers that allow to discriminate Tregs that develop in the thymus (tTregs) and Tregs that arise by conversion from CD4+Foxp3– Tconv in peripheral tissues (pTregs; ref. 32).