Of note, although one metastatic mixed mucinous BC displayed a dominant microsatellite instability mutational signature (Fig. 5b), it did not harbor genetic alterations in any of the core mismatch repair genes (Supplementary Table 4) and displayed a retained expression of MLH1, MSH2, MSH6, and PMS2 by immunohistochemical analysis (Supplementary Fig. 7a–, e). Here, MSH6 is linked to breast cancer.