The likely explanation is somatic mosaicism, where the first NF1 mutation occurs later in embryonic development between the blastocyst and the neuroblast stage in such a way that some cells are spared and remain wild type for NF1. In some occasions, the disease region may be so discrete that the NF1 diagnostic criteria cannot be met, which would explain the case of a single isolated neurofibroma that can be misinterpreted as a sporadic pNF. This evidence concerns the gene NF1 and neurofibroma.