SMARCB1 and schwannomatosis: Although the 4-hit/3-step model accounts for the majority of the tumours in the cases of SMARCB1 mutation-positive schwannomatosis, biallelic inactivation of the NF2 gene is not observed in all schwannomas (Boyd et al. 2008; Hadfield et al. 2008, 2010a; Sestini et al. 2008; Hulsebos et al. 2007, 2016).