Genetic testing in neurofibromatosis type 1 (NF1) occasionally reveals two heterozygous NF1 variants in the same individual. Correct interpretation hinges on allelic phasing, elucidation of somatic second hits in lesions, and distinction from bona fide dual RASopathy diagnoses. We provide a focused, critical review of the primary literature on (i) cis doublets in NF1, (ii) purported germline trans configurations—particularly in spinal neurofibromatosis, (iii) biallelic somatic inactivation of NF1 in café-au-lait macules, tumors, and juvenile myelomonocytic leukemia, and (iv) confirmed cases of dual RASopathy. We also include an index patient with two heterozygous NF1 variants to illustrate clinical challenges. Phase-proven cis doublets are rare, often closely spaced, and generally accompany classic NF1 without reproducible phenotype escalation. Claims of germline trans NF1 pathogenic variants weaken on reappraisal: second alleles reported in spinal neurofibromatosis typically do not meet current pathogenicity standards, while Nf1−/− mouse embryonic lethality underscores the biological implausibility of constitutive biallelic loss. In contrast, biallelic somatic NF1 inactivation is pervasive across NF1 lesions, including café-au-lait macules, plexiform and cutaneous neurofibromas, and juvenile myelomonocytic leukemia, due to mitotic recombination (copy-neutral loss-of-heterozygosity), microdeletions, or a second variant. Confirmed dual RASopathy diagnoses (most often NF1 plus PTPN11) are rare and produce blended systemic phenotypes. Interpretation of double NF1 findings should prioritize independent American College of Medical Genetics and Genomics/Association for Molecular Pathology classification of each variant, rigorous phasing (segregation/RNA/allele-specific methods), and tumor-focused analysis where applicable. Our index case and synthesis support a practical workflow that clarifies counseling and helps avoid over-calling dual pathogenicity.
Angelova-Toshkina et al. (2026) studied this question.