The adenomatous polyposis coli (APC) gene is a key tumor suppressor gene. Mutations in the gene have been found not only in most colon cancers but also in some other cancers, such as those of the liver. The APC gene product is a 312 kDa protein that has multiple domains, through which it binds to various proteins, including beta-catenin, axin, CtBP, Asefs, IQGAP1, EB1 and microtubules. Studies using mutant mice and cultured cells have demonstrated that APC suppresses canonical Wnt signalling, which is essential for tumorigenesis, development and homeostasis of a variety of cell types, such as epithelial and lymphoid cells. Further studies have suggested that APC plays roles in several other fundamental cellular processes. These include cell adhesion and migration, organization of the actin and microtubule networks, spindle formation and chromosome segregation. Deregulation of these processes caused by mutations in APC is implicated in the initiation and expansion of colon cancer.