Colorectal cancer is a major cause of morbidity and mortality. About 15 - 20% of all colorectal cancers are familial. Hereditary nonpolyposis colorectal cancer is an inherited cancer predisposition syndrome. It is caused by mutations in mismatch repair genes, predominantly MSH2 and MLH1. Although hereditary nonpolyposis colorectal cancer accounts for a minority of colorectal cancers, the mutations identified in these cases are important in our understanding of colorectal cancer pathogenesis. An increasing number of deletions in different exons of MSH2 and MLH1 genes is reported to result in hereditary nonpolyposis colorectal cancer. This paper reviews the genetics behind these genes and molecular study of the hereditary nonpolyposis colorectal cancer. This may help the medical professionals especially internists, gastroenterologists, and oncologists to update their knowledge in this field.