PIK3CD and breast cancer: Activating mutations of the catalytic subunit of PI3K (phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha [PIK3CA]), which belongs to the class IA PI3K family, are frequently observed in breast cancer [9, 10], and approximately 35–45% of cases of hormone receptor-positive breast cancer harbor mutations in this gene [11, 12].