ERBB2 and mismatch repair cancer syndrome 1: MMR deficiency is also present across all major breast cancer subtypes by immunohistochemistry in our cohort (1.6% in Luminal A, 2.4% in Luminal B, 1.5% in HER2 enriched, 0.5% in basal), illustrating that MMR deficiency testing is potentially relevant in all major breast cancer subtypes.