The discovery of a unique recurrent mutation in the gene encoding the general transcription factor GTF2I in thymomas of the A and AB subtypes7 (but also in a small fraction of B subtypes8) represents a major step forward towards the molecular characterization of these tumours; the mutation invariably affects leucine (L) residue 424, which is always converted to histidine (H), suggesting that this p.L424H missense mutation has tumour-promoting properties7. The gene discussed is GTF2I; the disease is neoplasm.