Diphtheria toxin A (DTA), a segment of the diphtheria toxin (tox), inhibits protein synthesis in cells. When released from a cell, DTA is nontoxic and cannot enter other cells independently without the help of diphtheria toxin B. In this study, we artificially synthesized the DTA gene sequence and cloned it into pEGFP-N1 to generate the recombinant vector pEGFP-N1-DTA. This recombinant vector was then transfected into 293T cells to observe the effect of DTA protein expression on enhanced green fluorescent protein (EGFP) protein expression and the proliferation of 293T cells. After 48 h, high levels of EGFP expression were seen in control pEGFP-N1-transfected cells, whereas very low levels were seen in cells transfected with pEGFP-N1-DTA. Reverse transcription polymerase chain reaction confirmed the expression of the DTA gene in cells transfected with pEGFP-N1-DTA. Further, the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay revealed a significant difference in cell proliferation between the control group and the pEGFP-N1-DTA-transfected group. Using the expression of EGFP expression as an indicator, this study revealed that DTA expression can inhibit intracellular protein synthesis and cell proliferation.