CHEK1 and neoplasm: While phosphorylation of histone H3 at serine 10 (H3 pS10), an epigenetic mitotic marker28, was found downregulated in tumor cells treated with leflunomide independent of the p53 status (Fig. 5d), its increased levels were observed in p53-deficient cells (4T1 cells and p53KO HCT116 cells) or p53-mutant MDA-MB-231 cells after Chk1 inhibition (Fig. 5d), indicating re-entry of these cells into the cell cycle.