Constitutional Mismatch Repair Deficiency (CMMRD) is a rare and devastating childhood-onset cancer predisposition syndrome caused by biallelic germline mutations in mismatch repair (MMR) genes. This inherited MMR deficiency causes an accumulation of errors mainly in repetitive regions of the DNA known as microsatellite instability (MSI). We have developed a valuable diagnostic tool that can detect MSI at high sensitivity in blood and oral mucosa samples from CMMRD individuals, robustly discriminating between CMMRD and other cancer syndromes with overlapping phenotype. Our approach allows a prompt CMMRD diagnosis that is essential for therapeutic decisions and clinical management of patients. (By Dr. Fátima Marín, https://jmg.bmj.com/content/early/2019/09/06/jmedgenet-2019-106272 )