People with a tumor predisposition syndrome have increased risks of various tumors. The cause is an inherited mutation in a certain gene, usually detected with a DNA-test using blood. Sometimes a novel mutation develops during early embryonic development. The mutation is then not present in all cells, which is called mosaicism. In mosaicism, a blood test may fail to detect the mutation, leading to a delayed diagnosis. This study shows that testing other tissues can identify the mutation and confirm the tumor predisposition syndrome. This provides clarity for affected individuals, improves cancer screening advice, and helps family members understand their own risks and options. (https://jmg.bmj.com/content/early/2026/02/17/jmg-2025-111224 )