Of the 16% of tested tumours found to be dMMR, only 51% were followed up as per diagnostic guidance: 121/372 (33%) patients with MSH2/MSH6/PMS2 deficient tumours had germline testing, and 1178/2190 (54%) tumours with MLH1 deficiency or MSI-High status had further somatic testing. The gene discussed is MSH6; the disease is neoplasm.