The tumor was hypermutated (56.7 somatic mutations per Mb, Table 2), showed the mutation signature of mismatch repair deficiency (Alexandrov et al., 2013), and had MSH‐2 and MSH‐6 protein losses as determined by immunohistochemistry (Fig. 4, Tables 2 and S1). Here, MSH6 is linked to mismatch repair cancer syndrome 1.