FL HO-1 knockout cells accumulate excess hemoglobin when induced to differentiate with EPO. FL cells were isolated from mouse embryos between days 12.5 and 13.5 and kept either in a nondifferentiated state (0 hours) or induced to differentiate during the indicated intervals. (A-B) FL/HO-1^−/− cells do not express HO-1 at mRNA or protein levels as determined by qRT-PCR and western blot analysis, respectively. (C) The color of cell pellets demonstrates that FL/HO^−/− cells exhibited a higher degree of redness than wild-type cells at 0 hours as well as at 24 and 48 hours of differentiation. (D) Measurement of hemoglobin levels in FL/HO-1^−/− cells before and after differentiation. Error bars of graphs represent standard deviation of 3 independent experiments (n = 3). *P < .05; **P < .001.