Although β-catenin/TCF4-mediated ABCB1 expression represents only one mechanism within the complex transcriptional regulation of the ABCB1 gene, our findings on the dominant action of gain-of-function β-catenin adds to the current knowledge on the role of ABCB1 in the biology of colorectal cancer. The gene discussed is ABCB1; the disease is colorectal cancer.