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Objectives/Hypothesis  Unilateral vestibular schwannoma (VS) occurs with a lifetime risk of around 1 in 1,000 and is due to inactivation of the  NF2  gene, either somatically or from a constitutional mutation. It has been postulated that familial occurrence of unilateral VS occurs more frequently than by chance, but no causal mechanism has been confirmed.    Study Design  Retrospective database analysis.    Methods  The likelihood of chance occurrence of unilateral VS, or occurring in the context of neurofibromatosis type 2 (NF2), was assessed using national UK audit data and data from the national NF2 database. Families with familial unilateral VS (occurrence in first‐ and second‐degree relatives) were assessed for constitutional  NF2  and  LZTR1  genetic variants, and where possible the tumor was also analyzed.    Results  Approximately 1,000 cases of unilateral VS occurred annually in the United Kingdom between 2013 and 2016. Of these, 2.5 may be expected to have a first‐degree relative who had previously developed a unilateral VS. The likelihood of this occurring in NF2 was considered to be as low as 0.05 annually. None of 28 families with familial unilateral VS had a constitutional  NF2  intragenic variant, and in nine cases where the VS was analyzed, both mutational events in  NF2  were identified and excluded from the germline. Only three variants of uncertain significance were found in  LZTR1 .    Conclusions  Familial occurrence of unilateral VS is very unlikely to be due to a constitutional  NF2  or definitely pathogenic  LZTR1  variant. The occurrence of unilateral VS in two or more first‐degree relatives is likely due to chance. This phenomenon may well increase in clinical practice with increasing use of cranial magnetic resonance imaging in older patients.    Level of Evidence  2b  Laryngoscope , 129:967–973, 2019

Familial unilateral vestibular schwannoma is rarely caused by inherited variants in the NF2 gene