IC261 is a selective, ATP-competitive CK1 inhibitor, with IC50s of 1 μM, 1 μM, 16 μM for Ckiδ, Ckiε and Ckiα1, respectively. IC261 is less active on PKA, p34, and p55 (IC50s > 100 μM). IC261 induces mitotic arrest, spindle defects and centrosome amplification in AC1-M88 cells. IC261 (1 μM) increases G2/M cells after 12 h, and causes cell death at 24 h in AC1-M88 cells. IC261 (1 μM) also induces apoptosis in the extravillous trophoblast hybrid cells. IC261 (1.25 μM) suppresses the proliferation of several pancreatic tumour cell lines, including ASPC-1, BxPc3, Capan-1, Colo357, MiaPaCa-2, Panc1, Panc89, PancTu-1 and PancTu-2 cells. IC261 (1.25 μM) specifically enhances CD95-mediated apoptosis of pancreatic tumour cells.