Constitutional mismatch repair deficiency (CMMRD) is an autosomal recessive cancer predisposition syndrome, causing café au lait macules, brain malformations and and multiple tumors, mostly leukemia/ lymphoma, brain, and gastrointestinal cancer in the first two decades. Early recognition of CMMRD may allow tailored surveillance and genetic counseling, also for heterozygote carriers who have Lynch syndrome. We report on five girls with the rare combination of pediatric systemic lupus erythematosus (pSLE) and CMMRD. In all patients, pSLE preceded the onset of malignancy. We discuss unique treatment dilemmas for these patients. pSLE in a young child with NF1 features should prompt testing for CMMRD. (By Dr. Yael Goldberg, https://jmg.bmj.com/content/early/2019/09/09/jmedgenet-2019-106303 )