In the early days of HER2 testing, amplification of the HER2 gene and the corresponding overexpression of HER2 protein was found in approximately 25 to 30 % of breast cancer [5], but this rate was probably an overestimate as it is now identified in approximately 15 to 20 % [6] of primary breast cancer cases, while recent data show a further decreasing trend to around 14 % [7]. Here, ERBB2 is linked to breast cancer.