The microsatellite instable hypermutated tumours are either associated with Lynch syndrome bearing a germline mutation in one of the mismatch repair proteins (MLH1, MSH2, MSH6 and PMS2) or tumours characterised by sporadic hypermethylation of the MLH1 promotor with subsequent blockade of the transcription process and loss of protein expression [6, 10]. The gene discussed is MLH1; the disease is Lynch syndrome.