Small molecule α isoform-specific PI3K inhibitors are currently in clinical trials in breast cancer (Hanker et al., 2019), with alpelisib (PIQRAY, BYL-719), in combination with fulvestrant, having been approved for treatment of hormone receptor (HR)-positive, HER2-negative, PIK3CA-mutated tumors (Brandao et al., 2019). Here, PIK3CB is linked to breast cancer.