ERBB2 and breast cancer: This categorization was developed using BC cell lines with known HER2 gene amplification status confirmed by FISH analysis (MCF7, BT20, T47D, MDA-MB-453, SK-BR-3, and BT474 ​cell lines) and a high concordance between IF intensity and FISH was reported, supporting the accuracy of the IF intensity as a proxy of HER2 expression level.