Abstract: Objective To identify the function and mechanism of MR1 in proliferation of BEL-7402 cells. Methods siRNA targeting MR1 and negative control siRNA were chemically synthesized, and transfected into BEL-7402 cells using Lipofectamine 2000. The silencing effect of MR1 siRNA was determined by semi RT-PCR. SRB assay, colony formation assay and growth curve assay were used to investigate whether MR1 siRNA regulated cellular proliferation. Cell cycles were assessed by flow cytometry. G2 arrest reagent nocodazole was used to show the possible effect of MR1 siRNA on G1 arrest. The expression of Cyclin D1 was determined by Western blotting. Results ①MR1 mRNA decreased significantly in BEL-7402 cells 24h after MR1 siRNA transfection. ②MR1 siRNA induced the down-regulation of cell growth. The expression level of Cyclin D1 in MR1 siRNA tranfected BEL-7402 cells decreased significantly by contrast to negative control siRNA transfected cells. ③Flow cytometry results showed that MR1 siRNA markedly decreased G2 phase population with nocodazole treatment, and distinctly increased G1 phase population. Conclusion The gene MR1 is involved in the proliferation of BEL-7402 cells. MR1 siRNA causes inhibition of the proliferation of BEL-7402 cells. One of the mechanisms of MR1 siRNA on the proliferation of BEL-7402 cells is the induction of G1 arrest.