Constitutional mismatch repair deficiency (CMMRD) is a very rare genetic condition conferring an extraordinarily high risk for development of cancer already during childhood and young adulthood. Patients with this cancer syndrome frequently have multiple café-au-lait spots and other signs that are indicative of a different, much more common condition, which is neurofibromatosis type 1 (NF1) caused by mutations in the NF1 gene. Recognising CMMRD in a child on the basis of NF1 features before cancer has developed provides the opportunity to start cancer surveillance early on. We developed guidelines that recommend when children suspected of NF1 but without an NF1 mutation should be tested for CMMRD.  (By Manon Suerink and Dr. Katharina Wimmer, https://jmg.bmj.com/content/early/2018/11/10/jmedgenet-2018-105664 )