The study will involve treating 29 subjects with methotrexate given intravenously. All subjects will receive treatment; there is no control arm or randomisation. The subjects will be known to have deficiency of MSH2, or a mutation (genetic change) in MSH2 that stops it functioning normally. This can either be demonstrated by testing for loss of MSH2 protein in the tumour itself, or by the demonstration of a mutation in the MSH2 gene in the subject's blood.