ERBB2 and breast carcinoma: In addition, in a large retrospective study investigating the reproducibility of HER2 IHC classification (0 vs 1+ vs 2+ vs 3 + ), although a substantial overall agreement was reported across 5 breast cancer-specialized pathologists (multi-rater overall kappa concordance Score=0.79), it should be noted that the discordancy was mostly driven by cases which were discordant between 0 vs 1 + (~43% of all discordant cases, 15% of total cases)13.