Wild-type PU.1 inhibits growth of leukemic cell colonies in agar. (A-C) Colony growth of cells from the indicated cell lines infected with vector expressing EGFP alone or EGFP plus either wild-type PU.1 or mutant PU.1. At 16 to 20 hours after infection, EGFP-expressing cells were sorted and plated at 100 and 50 cells per dish, as described in “Materials and methods.” Values shown are mean ± SD of quadruplicate plates and are representative of 2 to 4 experiments. (D) Western blot showing exogenous and/or endogenous PU.1 expressed in a leukemia cell line (X31.1) that had been infected with each of the indicated 3 viruses.