In fact, a link between Exo1 mutations and HNPCC has been questioned in subsequent studies, which have shown that a number of the Exo1 missense mutations identified by Wu et al. (20) are common variants in the human population (22) and that individuals with heterozygotic germline Exo1 deletions apparently do not develop HNPCC (23). Here, EXO1 is linked to hereditary nonpolyposis colon cancer.