To diagnose HER2-driven signaling dysfunction in breast cancer patients with normally expressed and non-amplified HER2, (Cheng et al. 2014; Cicenas et al. 2006; DiGiovanna et al. 2005; Hudelist et al. 2006; Kurebayashi et al. 2015; Ramic et al. 2013; Thor et al. 2000; Wulfkuhle et al. 2012) we developed the CELx HER2 Signaling Function (CELx HSF) Test, a novel assay that assesses dynamic HER2 signaling activity in live patient tumor cells. This evidence concerns the gene ERBB2 and breast cancer.