In patients with mosaic NF2, the ‘first-hit’ NF2 gene mutation is often present at a low level in blood cells and is sometimes only detectable in tumour tissue but not in blood cells (Evans et al. 1998, 2007; Kluwe and Mautner 1998; Kluwe et al. 2003; Paganini et al. 2014; Spyra et al. 2015; Smith et al. 2016). The gene discussed is NF2; the disease is neoplasm.