The use of neoadjuvant therapy (NAT) before surgery to render large tumours operable in breast cancer (BC) is increasingly common. However, only <30% of breast cancers treated with neoadjuvant chemotherapy would have a complete pathological response, which is the complete disappearance of cancer cells following treatment. Even with the addition of immunotherapy, the best response is only 60%. Although generally safe, immunotherapy can result in irreversible organ damage in a small population of patients. We have performed single cell sequencing on a unique clinical cohort to understand mechanisms of treatment resistance in all breast cancer subtypes. We have serial biopsy specimens from breast cancer patients at diagnosis, during chemotherapy and at surgery. These samples are subsequently characterised through single-cell sequencing analysis. So far, we have sequenced 23 pairs of samples with a total of 160,000 cells. We built a cellular landscape showing different compositions of cell populations in pre and post treatment samples. Intriguingly, in the post-treatment samples, we found that major cell types in tumour microenvironment, such as cancer-associated fibroblast (CAF) and perivascular-like (PVL) subpopulations shown different pattern of enrichment in responders and non-responders. Our sample collection and data analysis are still ongoing. Recently we have submitted additional 8 pairs of samples for sequencing. The data together will provide insights into understanding of breast cancer microenvironment and their role in the patients’ treatment response.