Recently, remarkable progress in molecular biology has enabled isolation of genes responsible for hereditary tumors such as retinoblastoma (RB), Wilms' tumor (WT), von Recklinghausen neurofibromatosis (NF 1), and familial adenomatous polyposis (FAP). Since patients with FAP develop multiple adenomatous polyps in the colon, some of which progress to colon cancer, isolation of the FAP gene allows us a rare opportunity to study genetic events underlying the well defined morphological changes during progression of colorectal tumors. In this report, we presented an approach called "positional cloning" which has become a powerful tool for identifying genes responsible for hereditary tumors, as well as characteristics of some of such genes.