Luminal cells (LCs) act as the cancer cell of origin in the most common BCa, the luminal subtype. It remains unclear whether the heterogeneity found in luminal-derived tumours and metastasis post treatment arises from a pre-existing heterogeneity within LCs. Recently developed technology allows for primary and metastasis BCa human/mouse organoid lines, which broadly recapitulate the diversity of the disease (Morphology, Histopathology, ER status, etc.) and are consistent with in vivo xeno- transplantations and patient responses. These will be used to identify in vitro drug resistance and clonal selection mechanisms and its relationship with LC heterogeneity. In this context, we will genetically define the various multipotent and differentiated populations using luminal markers and use lineage tracing to assess whether lineage-restricted tumor-initiating cells (TIC) maintain the ER+ lineage. We have devised a strategy based on editing the genomes of patient-derived tumor organoids using CRISPR/Cas9 technology to integrate reporter cassettes at desired marker genes. We aim to describe the gene programs that define these populations, and how they evolved upon treatment selection and metastasis. Lineage-tracing experiments will confirm the capacity of the distinct populations to self-renew and generate progeny over periods of metastasis and drug treatment selection.