We have isolated several genes on chromosome 5q21 region tightly linked to hereditary familial polyposis coli (FAP) and Gardner's syndrome (GS). Two of these genes (MCC and APC) were found to be somatically altered by point mutation, deletion or insertion in tumors of sporadic colorectal cancer patients. One (APC) of them was also found mutations in the germ line of both APC and GS patients. The identification of these genes has significant implications for understanding the pathogenesis of colorectal neoplasia and for the diagnosis and counseling of individuals with inherited predispositions to colorectal cancer.