People, who carry a mutation in a DNA mismatch repair (MMR) gene i.e. people with Lynch syndrome, have a high risk of bowel, uterus and several other cancers. The proportion of these people whose mutation is de novo (not inherited from either parent) is not known. In this paper, we report six bowel cancer cases with de novo mutations in MMR genes (two in MLH1, three in MSH2, and one in MSH6) and estimate the frequency of MMR mutations that are de novo to be between 1 in 100 (1%) and 1 in 20 (5%). De novo MMR gene mutations are uncommon causes of Lynch syndrome. (By Aung Ko Win MBBS, MPH, http://jmg.bmj.com/content/early/2011/06/02/jmedgenet-2011-100082.abstract?papetoc )