The aim of the proficiency testing scheme “PIK3CA mutations in HR+/HER2-breast cancer” was the quality assessment of validated, correctly performed, and reproducible PIK3CA mutation testing by the participating institutes without methodological restrictions including locally validated laboratory-developed tests (LDTs), as well as research use only (RUO) and CE-IVD (CE in vitro diagnostics) assays. This evidence concerns the gene ERBB2 and breast carcinoma.