Currently, only a few validated biomarkers and models exist guiding or predicting treatment response for specific patient groups. Therefore, a patient-tailored clinical model needs to be developed to address tumour heterogeneity and thereby guide treatment selection on an individualized level. Organoids are patient-derived aggregates that grow in 3D and maintain self- renewal pluripotency and lineage-specific differentiation. Therefore, in contrast with conventional cell lines, they are thought to sustain patient heterogeneity and characteristics and are consequently already in use for drug response screening. This now offers the opportunity to investigate if the primary patient breast cancer organoid platform reflects disease progression, treatment response and relapse in patients with different clinical breast cancer subtypes. Goal: To develop a living biobank from prospective patient-derived breast cancer tissue. The questions we will address are: 1. Do patient-derived breast cancer organoids retain the clinical behaviour and characteristics of the primary patient tumour? 2. Can BC organoids be used to derive prognostic and predictive biomarkers to inform treatment decisions? 3. Can the investigators utilize BC organoids to discover novel actionable targets and combination treatments for therapeutic intervention for breast cancer patients? 4. Can BC organoids be used to discover mechanisms of treatment resistance and relapse?