Knock-down of Spi-1/PU.1 in spi-1 transgenic proerythroblasts. siRNA expression was induced by a 24-hour treatment with 100 ng/mL dox (+) in cells expressing one siRNA (A1 and A2) or 2 siRNA (A2B and A2C) or the empty vectors (C). Untreated cells were grown without dox (−). Spi-1 expression was analyzed by Western blotting. Three bands corresponding to Spi-1 protein were detected. Equal loading was verified by β-actin. The percentage of Spi-1 expression was calculated relatively to untreated cells by taking into account the 3 bands. C stands for control cells.