Mechanisms for USP39 regulating tumor growth. Western blot analysis was conducted to detect the expression of cell cycle checkpoint proteins FoxM1, PLK1, cyclin B1 and CENPA. (A) FoxM1 was significantly decreased following USP39 knockdown in SMMC-7721 cells at the mRNA level and markedly decreased at the protein level. (B) Expression of PLK1, cyclin B1 and CENPA were downregulated following USP39 knockdown in SMMC-7721 cells. Fox M1, Forkhead Box M1; PLK1, polo-like kinase 1; CENPA, centromere protein A; USP39, ubiquitin specific peptidase 39. **P<0.01 vs. SMMC-7721/KD.