The recent discovery of a new mutation in the DNA mismatch repair gene PMS2 that is recurrent in the Inuit population surrounding Hudson Bay, Canada offers new insights suggesting that even a small, residual function of the gene in patients is sufficient to delay the age of onset of tumours. This new finding adds to existing knowledge that the disease is more severe in individuals carrying two mutated copies of PMS2 than in individuals with only one mutated copy, and could serve as a model from which to design new preventive therapies in families with defects in these genes. （By Nancy Hamel, http://jmg.bmj.com/content/early/2015/02/17/jmedgenet-2014-102934 ）