RK-287107 is a novel tankyrase-specific inhibitor that inhibits tankyrase-1 and -2. RK-287107 causes Axin2 accumulation and downregulates β-catenin, T-cell factor/lymphoid enhancer factor reporter activity and the target gene expression in colorectal cancer cells harboring the shortly truncated APC mutations. Consistently, RK-287107 inhibits the growth of APC-mutated (β-catenin-dependent) colorectal cancer COLO-320DM and SW403 cells but not the APC-wild (β-catenin-independent) colorectal cancer RKO cells.