PIK3CA and breast cancer: Activation mutations in PIK3CA, the gene encoding the catalytic subunit of PI3K, p110a, occur in 48% of invasive lobular carcinoma [8] and in 41%, 40%, 31%, and 14% of hormone-positive HER2-negative (HR+HER2), HR+HER2+, HR-HER2+, and triple-negative breast cancer (TNBC) breast cancer, respectively [9].