Given that no somatic mutations of PIK3CD gene encoding the PI3K p110δ have been reported in breast cancer6,14–17, we sought to explore the p110δ protein expression during human breast cancer progression by immunohistochemistry in a collection of human breast carcinomas of grade I (n = 20), grade II (n = 20) and grade III (n = 20). This evidence concerns the gene PIK3CD and breast cancer.