Expression of gp91^phox and associated NADPH oxidase enzyme activity is restored by PU.1 transduction of 503 cells. (A) RT-PCR analysis of cells revealed gp91^phox mRNA in normal neutrophils and PU.1-transduced 503 cells (503-PU), but not in 503 as previously documented or in G-CSF receptor-transduced 503 (503-GR). The far lefthand lane contains a 100-bp DNA ladder. (B) An assay for cytochrome c reduction in response to PMA, which measures O₂⁻ production, was performed to address the functionality of the enzyme NADPH oxidase, of which gp91^phox is a principal subunit. Detectable production of O₂⁻ was found in normal neutrophils (NORMAL) and 503-PU, but not in 503 or 503-GR. SOD, superoxide dismutase.