Several errors occurred in the leading article by Frayling (Gut1999;45:1–4). On page 2, column 2, paragraph 1, “If it pairs with thymine, a G6A mutation will result” should read “If it pairs with thymine, a G→A mutation will result”. Page 2, column 2, paragraph 2, “However, it may help us understand why loss of MMR is advantageous to a tumour cell, although there is an indication why the loss of one MMR allele might be an advantage” should read “However, it may provide a bridge in our understanding as to why the loss of MMR is actually an advantage to a tumour cell, though there are no clues, as yet, as to why loss of one MMR allele might be advantageous”. Page 3, column 2, final paragraph, “Early studies used a bank of up to a dozen different microsatellites, mostly (CA)n repeats, which were often chosen semi-randomly and carefully to avoid issues of bias due to allelic loss in tumours” should read “Early studies used a bank of up to a dozen or so different microsatellites, mostly (CA)n repeats, often chosen semi-randomly, sometimes chosen carefully to avoid issues of bias due to allelic loss in tumours”.