ERBB2 and breast cancer: PIK3CA mutations are particularly common in early breast cancer, present in up to 47% of hormone receptor-positive (HR+)/HER2– luminal A tumors, 33% of HR+/HER2+ luminal B tumors, 39% of HER2-enriched tumors, and 8–25% of basal-like or triple-negative tumors (22, 24, 25).