AIM: Tyrosinase gene was transfected into HEK293 cell as a reporter gene, it's property of synthesizing melanin, which can be examined by magnetic resonance imaging(MRI), is used to evaluate the tyrosinase gene's expression. The aim of this study was to search a way to evaluate the results of gene expression by MRI in vitro .METHODS: The plasmid of pcDNA3tyr which carried the full-length cDNA of tyrosinase gene was transfected into HEK293 cell by lipofectin. To observe the MRI signals of expressed melanin,the transfected cells were scanned by MRI sequences of T 1WI, T 1WI/SPIR and T 2WI. On the other hand, fontana stain was used to search for melanin granules in transfected cells, RT-PCR method was used to search for cDNA of tyrosinase gene. RESULTS: (1) Plasmids of pcDNA3tyr could be transfected into HEK293 cells and could synthesize a large amount of melanin. The synthetic melanins of 10 6 cells, which had been transfected 5?g, 10?g, 20?g plasmids of pcDNA3tyr separately, were all sufficient to be detected by MRI and appeared high signal in MRI T 1WI、T 1WI/SPIR、T 2WI sequences. The signal intensities of MRI imaging were related to the amounts of transfected plasmids positively. (2) The melanin granules could be found in HEK293 cells by Fontana stain. (3) The cDNA fragment of tyrosinase gene could be detected in transfected HEK293 cells by RT-PCR. CONCLUSION: The fact that MRI could detect the synthetic melanin of HEK293 cells, which controlled by expression of exogenous gene, demonstrates that medical imaging connecting with molecular biology technology can evaluate the result of gene expression in vitro .