• mosaicism
• neurofibromatosis 2
• NF2 mutations
• vestibular schwannoma

Neurofibromatosis 2 (NF2), an autosomal dominant disorder that is characterised by tumours of cells of neural crest origin, is caused by inactivating mutations of the NF2 gene on chromosome 22q12.^1,2 Bilateral vestibular schwannomas are the most frequent manifestation of the disease, but other central and peripheral nervous system schwannomas, cerebral meningiomas, and ocular abnormalities are also common.^3–5 The birth incidence of NF2 is 1 in 33 000 to 1 in 40 000.⁶ Constitutional NF2 mutations have been found in 30-60% of NF2 patients, and genotype-phenotype correlations have been substantiated.^7–12

Half of NF2 patients are new mutations,⁶ in whom constitutional NF2 mutations can occur either in parental germline cells (prezygotic) or in cells after fertilisation (postzygotic). Postzygotic mutations can result in mosaicism, defined as the presence of a mutation, deletion, or chromosomal abnormality in a group or population of cells.¹³ Owing to the rarity of NF2, most previous reports of mosaicism in NF2 have been limited in scope,^14–16 although a recent study estimated that 25% of NF2 patients with new mutations are mosaic.¹⁷ In this study, we describe mutational analysis of 27 mosaic cases of NF2 and the results of genetic testing in their children.