MCM9 and cancer: This difficulty arises mainly from the limited inclusion of MCM8/MCM9 genes in current diagnostic gene panels for cancer and polyposis, constraining our case series, and the relative rarity of germline MCM8/MCM9 variants in the general population, as reflected by our investigations in gnomAD version 2.1.1, the 100000 Genomes Project, and the 200,000 exome release of the UK Biobank.