A-66 is a potent, selective inhibitor of the PI3K isoform p110α (IC50 = 32 nM in a cell-free assay). It displays over 100-fold selectivity for p110α over other isoforms. A-66 is effective in vivo, suppressing the growth of SK-OV-3 tumor xenografts in mice. It also impairs all measures of in vivo insulin action in mice. A-66 partially suppresses B cell receptor-dependent Akt activation and proliferation.