ESR1 and neoplasm: To investigate the global variations in breast heterogeneity among different models, we examined scRNA‐seq data that included transcriptional profiles from normal breast tissue, preneoplastic BRCA1+/– tissue, primary tumour samples from three clinical subtypes (ER+, HER2+, and TNBC),26 patient derived xenografts (PDXs), PDX‐derived organoids (PDXOs), cell lines, and several matched primary tumour associated lymph nodes.