CHEK1 and breast carcinoma: To establish the effect of combined DHODH/Chk1 inhibition on tumor growth in vivo, FVB/N c-neu mice were injected with murine breast cancer NeuTL p53-deficient cells and administered with leflunomide and the Chk1 inhibitor intraperitoneally twice a week for 2 weeks (see Methods for details).