Ligand-independent growth due to KITD814V expression in wild-type HSC/Ps is abrogated in the absence of p85α subunit of class IA PI3K, but not in the absence of p85β subunit of class IA PI3K. (A,B) Vector- or KITD814V-expressing wild-type, p85α^−/−, or p85β^−/− HSC/Ps were subjected to a thymidine incorporation assay in the absence of cytokine stimulation. After 48 hours of culture, cells were pulsed with [³H] thymidine for an additional 6 hours. Bars denote the mean thymidine incorporation (cpm ± SD) from 1 of at least 5 independent experiments performed in quadruplicate. *P < .05, KITD814V-expressing p85α^−/− cells versus wild-type cells. (C,D) Vector- or KITD814V-transduced cells were subjected to an in vitro methylcellulose progenitor colony assay in the absence (C) or presence (D) of SCF 100 ng/mL. Colonies were counted after 10 days of culture. Bars denote the average number of colonies ± SD from 1 of 2 independent experiments performed in triplicate. *P < .05, KITD814V-expressing p85α^−/− cells versus wild-type cells.