In light of recent clinical success of copanlisib (PI3K-α/δ inhibitor) in treating breast cancer and other solid tumors, we examined antitumor efficacy of a panel of single and dual isoform-specific PI3K inhibitors including idelalisib (PI3K-δ), copanlisib (PI3K-α/δ), duvelisib (PI3K-γ/δ), alpelisib (PI3K-α), and AZD8186 (PI3K-β/δ). This evidence concerns the gene PIK3CB and breast cancer.