Expression of PI3K isoforms. Lysates or immunoprecipitates (IP) from wild-type and Pik3r1 null (WT and KO) cells transformed by v-ABL or BCR-ABL were immunoblotted with the indicated antibodies. (A) p110α was detected by IP and immunoblotting. (B) p110β, p110δ, p85α, p85β, p55α, and p50α were detected by direct immunoblotting; β-actin was used as a loading control. (C) To optimize detection of p85β with anti-pan-p85, lysates were first subjected to partial immunodepletion of p85α using a specific mAb. p55γ was detected by direct immunoblotting; β-actin was used as a loading control. Isoform blots are representative of 2 to 6 cell lines per genotype.