Objective:To prepare bone marrow mesenchymal stem cells (BMSCs) transfected with therapeutic gene early growth reactive protein 1 (Egr1)-sodium/iodine symporter (NIS) and carrying gold nanoparticles (AuNPs), and to investigate the feasibility of Egr1 in promoting NIS expression and the radiasensitization effect of AuNPs.Methods:BMSCs transfected with lentivirus(Lv)-Egr1-NIS-cytomegalovirus(CMV)-green fluorescent protein (GFP) in the experimental group and Lv-Egr1-GFP in the control group were prepared and the expression of NIS induced by radioiodine was verified by iodine uptake determination. The optimal incubation time and concentration of AuNPs were observed with laser confocal microscopy. The cytotoxicity of AuNPs suspension liquid was investigated using cytotoxicity test. Iodine uptake assay was performed to investigate NIS gene expression of BMSCs-Egr1-NIS incubated with AuNPs. In vitro chemotaxis of BMSCs-Egr1-NIS incubated with/without AuNPs to breast cancer cells were verified by cell migration experiment. The radiosensitization effect of AuNPs for 131I on killing breast cancer cells MDA-MB-231 were explored. The one-way analysis of variance and Dunnett t test were used for data analysis. Results:BMSCs-Egr1-NIS (unstable transformation) was successfully prepared. Egr1 could promote NIS expression with the induction of radioiodine. The iodine uptake capacity in BMSCs-Egr1-NIS increased by 2.5-5 times or even higher compared with BMSCs-Egr1-GFP. The better incubation conditions of AuNPs for BMSCs phagocytosis were 0.20 g/L(24 h) or 0.10 g/L(48 h). The cytotoxicity of AuNPs was low in appropriate incubation time and concentration, and there was no effect on iodine uptake and chemotaxis. The chemotaxis to MDA-MB-231 of BMSCs-Egr1-NIS was identified. AuNPs radiasensitization assay showed that absorbance ( A) 570 nm of MDA-MB-231 cells were significantly deferent in 131I killing groups and blank control group without 131I ( F=60.670, P<0.01), and the cytotoxicity of 131I to MDA-MB-231 cells in the 131I killing groups with 0.20 g/L AuNPs and 0.40 g/L AuNPs ( A570 nm values: 0.87±0.05, 0.41±0.07) was significantly higher than that in the group with 0 g/L AuNPs ( A570 nm=1.39±0.11; both P<0.01). Conclusions:BMSCs, transfected with therapeutic gene Egr1-NIS and incubated with AuNPs, can be used as a carrier to target breast cancer. NIS gene expression of BMSCs-Egr1-NIS was highly promoted in the presence of radioiodine. At the same time, AuNPs can be used as a radiation sensitizer for 131I treatment.