Phenotypic similarities between haematological tumour cells and normal haematopoiesis are key to classifying haematological malignancies, and this requires characterization of cells during normal haematopoiesis. Santagata et al. set about characterizing the cell types in normal breast epithelium to see whether they could use this information to classify breast tumours. Using 15,000 normal breast cells and breast epithelial markers, they identified 11 differentiation states, which were used to classify breast tumours into four subtypes based on the expression of certain receptors. These subtypes were distinct from standard molecular subtyping and they could be used to refine the classification system.