Microsatellite instability (MSI) and POLD1 mutations are usually described in colorectal tumours in patients with polyposis syndrome but rarely found in breast tumours. This case describes a metastatic luminal B breast tumour in a young patient with an important family history of cancer. Mutational studies found a Variant of Uncertain Significance (VUS)-type alteration in POLD1 that motivated the study for MSI, which was found positive. Recent data point towards the use of pembrolizumab as a treatment option for tumour presenting with MSI instead of chemotherapy.
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