Hereditary non-polyposis colorectal cancer (HNPCC) is a type of hereditary colorectal cancer with autosomal dominant traits. Its causal genes are mismatch repair genes such as the hMSH2 and hMLH1 genes. Owing to its frequency, juvenile onset, and the outbreaks of multiple colorectal cancers and cancers occurring over multiple organs, it is recognized as a very important disease for the purpose of understanding not only colorectal cancers, but also other digestive cancers, and furthering pathological inquiry into the state of cancers in general.