Pulsed field gel electrophoresis was carried out on lymphocyte DNA obtained from ten patients with the inherited premalignant condition adenomatous polyposis coli and a similar number of control samples to search for large scale molecular rearrangements in the vicinity of the APC locus. DNA from one patient showed a rearranged pattern which was interpreted as evidence for an inversion of approximately 150 kb, changing the transcriptional orientation of the neighbouring MCC gene and bringing it closer to APC. Such an interpretation was supported by fluorescent in situ hybridization data.