In a Europe-wide external quality assessment, we compared performance of an mRNA-based method [Xpert® Breast Cancer STRAT4 (CE-IVD)] for determining ESR1, PGR, ERBB2, and MKI67 expression against the gold standard [immunohistochemistry (IHC)/HER2 in situ hybridization (ISH)]. This evidence concerns the gene ESR1 and breast cancer.