Mosaic neurofibromatosis 1 (MNF1) or mosaic neurofibromatosis (previously designated as neurofibromatosis type V and segmental neurofibromatosis), is a segmental or quasi-segmental phenotype of NF1 caused by a postzygotic mutation of the NF1 gene, resulting in mosaicism. Patients with this condition lack a family history of the disease. MNF1 presents with features of NF1 in localized areas of the body. It may involve one segment only (localized) or multiple segments, even in up to half the body (generalized). It can be either symmetrical or asymmetrical depending on the cell lines affected and the timing of the mutation. The incidence is estimated to be 1 in 36 000-40 000 but is likely higher as it is probably underreported.

MNF1 results from a mutation of the NF1 gene located on the long arm of chromosome 17. This gene locus encodes for neurofibromin, which functions as a tumor suppressor gene. Clinically, the most common signs of MNF1 are pigmentary changes (café au lait macules, freckling, diffuse background pigment), cutaneous neurofibromas, and plexiform neurofibromas. Lisch nodules and optic gliomas are uncommon. While up to 30% of patients with MNF1 will have extracutaneous manifestations of NF1, including hypertension, learning disabilities, and seizures, patients with MNF1 will generally have milder clinical manifestations and fewer generalized complications compared to those with inherited NF1.

It is important to note that gonadal mosaicism can occur in MNF1 even in the absence of inguinal freckling, resulting in a small but real risk of complete NF1 in some offspring.