The adenomatous polyposis coli (APC) gene product is mutated in the vast majority of human colorectal cancers. APC negatively regulates the WNT pathway by aiding in the degradation of β-catenin, which is the transcription factor activated downstream of WNT signaling. APC mutations result in β-catenin stabilization and constitutive WNT pathway activation, leading to aberrant cellular proliferation. APC mutations associated with colorectal cancer commonly fall in a region of the gene termed the mutation cluster region and result in expression of an N-terminal fragment of the APC protein. Biochemical and molecular studies have revealed localization of APC/Apc to different sub-cellular compartments and various proteins outside of the WNT pathway that associate with truncated APC/Apc. These observations and genotype–phenotype correlations have led to the suggestion that truncated APC bears neomorphic and/or dominant-negative function that support tumor development. To analyze this possibility, we have generated a novel allele of Apc in the mouse that yields complete loss of Apc protein. Our studies reveal that whole-gene deletion of Apc results in more rapid tumor development than the APC multiple intestinal neoplasia (Apc Min) truncation. Furthermore, we found that adenomas bearing truncated Apc had increased β-catenin activity when compared with tumors lacking Apc protein, which could lead to context-dependent inhibition of tumorigenesis.