) immune system, geneexpression was analyzed in different developmental stages and tissues. Four tissues of grass carp at different timesafter challenge with the pathogenic bacterium were analyzed by real-time quantitativeRT-PCR. The 642 bp Open Reading Frame (ORF) region of grass carp was expressed using prokaryoticexpression technology. The fusion protein had a relative molecular mass of 40 kD. Real-time quantitative RT-PCRanalysis indicated that the mRNA level in embryos gradually increased during embryogenesis from cleavageto fry stages after fertilization, with the highest level in the fry stage. In adult fish, the transcripts were found to be highly abundant in liver; moderatelyabundant in the head kidney, spleen, and intestine; and least abundant in the eye and bladder. After challenge withmRNA level in the head kidney, liver, spleen, andintestine significantly increased (expression in the liver was significantly increased at 12 hpost-treatment; followed by a decrease at 24 h post-treatment; another increase at 48 h, with a maximum level at72 h post-infection; and the high expression level was maintained at 96 h and 7 d post-treatment. There was significantup-regulation in the intestine at 12 h post-treatment, with a maximum level at 48 h post-treatment; significantdown-regulation at 72 h and 96 h; and the expression regressed to the initial level by 7 d. In the spleen,mRNA level increased at 4 h post-treatment, with a maximum level at 12 h; then, there was significantdown-regulation at 24 h and 48 h; and the expression regressed to the initial level by 7 d. Bacterial infection canincrease plays an important role inthe innate immune response of grass carp.