As discordance in HER2 status between primary and recurrent breast cancer has been documented [51,52,53], and molecular heterogeneity associated with differential survival time after treatment with trastuzumab exists within the HER2+ population [54,55], retesting metastatic breast cancer for HER2 status, even in the HER2-low category, and molecular characterization of residual disease are gaining increasing attention [56,57]. The gene discussed is ERBB2; the disease is breast cancer.