APC mutations initiate most colorectal cancers (CRCs), but cellular mechanisms linking this to CRC pathology are unclear. We reported that wild-type APC in the colon down-regulates the anti-apoptotic protein survivin, and APC mutation up-regulates it, explaining why most CRCs display survivin overexpression and apoptosis inhibition. However, it does not explain another hallmark of CRC pathologyincreased mitotic figures and cell proliferation. Because survivin activates aurora-B kinase (ABK) in vitro, catalyzing mitosis, we hypothesized that in normal colonic crypts, APC controls ABK activity, while in neoplastic APC-mutant crypts, ABK activity is up-regulated, increasing mitosis. We quantitatively mapped intracryptal distributions of survivin, ABK, and markers of activated downstream signaling and mitosis (INCENP, phospho-histone-H3, phospho-centromere-protein-A). In normal crypts, gradients for these markers, ABK:survivin:INCENP complexes, and ABK activity were highest in the lower crypt (inverse to the APC gradient). In neoplastic crypts that harbor Although several lines of evidence indicate that a mutation at the APC locus initiates most cases of colorectal cancer (CRC), much less is known about the subsequent molecular and cellular mechanisms that link this mutation to the pathophysiology of colon tumorigenesis. Investigating this link by studying the anti-apoptotic protein survivin, we found that wild-type APC down-regulates survivin expression 1 and mutation of APC up-regulates it in mouse 2 and man. 3 While this might explain why most colon tumor cells show increased survivin expression and inhibition of apoptosis, it does not explain the increased mitotic figures and cell proliferation that are also pathological hallmarks of tumors. Since experiments using cultured cells have shown that survivin activates ABK, 4,5 which catalyzes mitosis, and since several lines of evidence suggest that ABK is involved in tumorigenesis, 6 -10 we hypothesized that: (i) in normal human colonic crypts wild-type APC down-regulates ABK