ERBB2 and cancer: In summary, we have developed a metric based on whole transcriptome data that can stratify LumA cancers based on subtype purity and thus provide information that is potentially predictive with respect to prognosis and treatment response, as exemplified by the recently reported results from a trial of trastuzumab deruxtecan in patients with “HER2-Low” and predominantly ER-positive breast cancer [31].