IC261 is a reversible, ATP-competitive inhibitor of casein kinase 1 (CK1) that inhibits CK1δ and CK1? (IC50 = ~1 μM for both), as well as CK1α (IC50 = 16 μM). It is at least 100-fold less effective against PKA, p34cdc2, and p55fyn. IC 261, at 1 μM, inhibits cytokinesis in primary mouse embryo fibroblasts. IC261 is used to elucidate the role of CK1 in cells and in whole organisms.
