NFATC1 and acute lymphoblastic leukemia: As Nfatc1 P1 activity is initiated only at the pTCR-positive thymocytes and pTCR signals induce NFATc1 activation42, this is most likely the natural-feedback mechanism to suppress pTCR expression and thereby prevent T-ALL development once the cells have received optimal pTCR signals (Supplementary Fig. 8c).