The discovery of a tumor suppressor gene opens a new pathway to discovery of the fundamental mechanisms that underlie tumor initiation and progression. An inherited tumor suppressor gene is of special interest in that it defines a step in the tumorigenesis pathway that can be rate limiting in development of that tumor type. In the case of colon cancer, we were fortunate in identifying an inherited tumor suppressor gene, the APC gene, that plays a major etiologic role in both the inherited disease, familial adenomatous polyposis (FAP), and in sporadic colon polyps. Characterization of the molecular biology of that gene, and the underlying mechanisms that result in the development of colon tumors, could provide new approaches to both colon cancer diagnostics, therapeutics and chemopreventives. We have embarked, therefore, on a series of exploratory studies designed to provides clues to possible functional roles for the APC protein. We have found through immunocytochemistry that APC protein is distributed throughout the cell, in both the cytoplasm and nucleus. Furthermore, within the nucleus much of the APC protein seems associated with the nucleoli. The cytoplasmic label is distributed in a punctate pattern, with concentrations at the leading edge of migrating cells at the ends of microtubules. Furthermore, following an extraction of the cells that leaves behind primarily cytoskeletal and nuclear scaffold structures, we see strong APC staining of these structures. The yeast two-hybrid system has offered a number of potentially interacting partners for APC, including a new binding site for alpha-tubulin. These results, and others recent discoveries concerning APC, suggest a rather global role for APC protein, modulating cellular activity and signal transduction pathway from the cell periphery to the nucleus.