Molecular tests, such as the 70-gene signature (MammaPrint, Agenda BV, The Netherlands) and the 21-gene assay (Oncotype DX, Genomic Health, United States), have shown to improve clinical decision making in early-stage breast cancer of certain molecular and clinical subtypes, such as oestrogen-positive (ER+) or HER2-negative (HER2−) breast cancer [7, 8]. Here, ERBB2 is linked to breast cancer.