SR-3029 is a potent CK1δ/CK1ε inhibitor, with IC50s of 44 nM and 260 nM, respectively. SR-3029 is ATP competitive, with Kis of 97 nM for CK1δ/CK1ε. SR-3029 also blocks CDK6/cyclin D3, CDK6/cyclin D1, CDK4/cyclin D3, CDK4/cyclin D1 and FLT3, with IC50s of 427, 428, 368, 576, and 3000 nM, respectively. SR-3029 shows inhibitory effects on A375 cells, with an EC50 of 86 nM. CK1δ is a necessary and sufficient driver of Wnt/β-catenin signaling in human breast cancer. SR-3029 shows less potent activities against MCF7 and T47D breast cancer cells and the MCF10A cell line, which express low amounts of CK1δ.