E47-ER-Tac and bHLH-ER-Tac retroviral constructs were described (Sayegh et al., 2003). Virus was generated by transfection of the constructs and packaging vector into HEK 293T cells. E2A–deficient hematopoietic progenitor cells were transduced as described previously (Quong et al., 1999). After 18 h, E47-ER and bHLH-ER were activated for 6 hr with 1 µM 4-hydroxy-tamoxifen and transduced cells were magnetically enriched (Miltenyi) for bi-cistronically co-expressed human CD25 (TAC antigen).
