To investigate and quantify the heterogeneity of HER2+ carcinomas by using single cell investigation, we performed detailed in situ analyses on samples from a Norwegian observational study (RA‐HER2), comprised of 37 HER2+ patients treated in a neoadjuvant setting with trastuzumab and chemotherapy where both response data and clinical follow‐up were available. This evidence concerns the gene ERBB2 and carcinoma.