Neurofibromatosis type 1 (NF1) is a genetic disorder characterised by a range of mostly benign tumours, including cutaneous neurofibromas. In many cases, the pathogenic variants in the NF1 gene arise de novo, meaning they are not inherited. In some instances, these variants are present only in a subset of cells, a condition known as genetic mosaicism. Mosaic NF1 often presents with a milder clinical phenotype and poses challenges in diagnosis and genetic counselling, as conventional blood-based genetic testing may fail to detect the pathogenic variant. This paper discusses five such cases and proposes a comprehensive diagnostic approach, emphasising tumour-derived DNA analysis as an effective method for detecting mosaic cases. (By Dr. Tabea Hartung, https://jmg.bmj.com/content/early/2025/03/26/jmg-2024-110580 )