The severity of the phenotype of NF2 SWN5 depends, in part, on the type of PV in the NF2 gene, with whole gene deletions and missense variants resulting in a milder phenotype (later onset of symptoms and lower tumour burden), compared with truncating and frameshift variants, which cause the severe phenotype characterised by early onset of symptoms and high tumour burden6–9. This evidence concerns the gene NF2 and acquired polycythemia vera.