Fish are an ideal model for research on functional genes. Transgenic fish have many important applications in aquaculture. Recently, a number of researchers have evaluated the use of transgenic fish to improve the commercial value of a species by manipulating traits such as growth, sex, and disease resistance. In particular, the function of sex-determining genes has received significant attention. The current studies of gene function, which are related to sex differentiation, are largely confined to a few fresh water fish species. In contrast, little effort has been made to study gene function in marine fish because of logistical difficulties, such as the presence of a hard fertilized egg membrane, low survival rate, and/or long period of a generation. To address this gap, we studied the function of a homologous gene in a model marine fish species. A number of methods have been used to modify the fish genome, among which microinjection is the most commonly used approach. We constructed expression vectors by subcloning Cinto a pIRES- hrGFP-1α vector. in Medaka was induced by injecting these GFP constructs into the cytoplasm of fertilized Medaka embryos at the one cell stage. We then measured GFP expression and hatching rate. Additionally, we quantified the level of gene integration, expression, and the effect on expression. GFP was expressed at high levels 48 h after injection and the number of embryos expressing GFP decreased with the development of embryos. The hatching rate and integration efficiency of embryos injected with the homologous expression vector was significantly higher than those injected with the heterologous . Furthermore, in fishes with was down-regulated. Our research provides a foundation for the study of sex differentiation and contributes to our knowledge of sex-determining genes in marine fish.