We describe a young woman with a mosaic form of neurofibromatosis type 1 (MNF1). In mosaicism, a genetic change occurs after conception and is therefore present in only some cells of the body rather than all cells. As a result, standard blood tests may miss the genetic change, especially in people who only have limited skin findings such as café-au-lait spots or freckling. In this patient, we analyzed pigment-producing skin cells and used an advanced DNA technique called optical genome mapping to identify a rare chromosome rearrangement affecting the NF1 gene. This study highlights how mosaicism can complicate diagnosis and shows a promising new approach for confirming MNF1, improving patient care and genetic counselling. (https://jmg.bmj.com/content/early/2026/05/17/jmg-2026-111525)