The purpose of this study is addressed the function of Ad4BP/SF1 in developing spleen of Ad4BP/SF-1 KO mouse. I have modified the BAC-Ad4BP endogenous by homologous recombinant with modification cassette-containing tetracycline transactivator system and LacZ reporter genes to allow the expression of Ad4BP/SF1 in developing tissues and organ differentiation, regulate of Ad4BP/SF1 mechanism, and reveal the defect due to Ad4BP/SF1 deficiency. Recently studies, by immunihistochemistry of adult tissues showed that revealed coincident expressing between Ad4BP/SF-1 and lacZ, indicating the tTA system seemed to reproduces the endogenous expression of Ad4BP/SF-1, and thus likely to lacZ, Ad4BP/SF-1 in the BAC transgene was expected to be expressed in the corresponding tissues under the control of the bidirectional promoter. Interestingly, the BAC-Ad4BP-tTA transgenic mice were succeeded to up regulate the expression of Ad4BP/SF-1 at the protein level than wild type. And the fetal spleen of Tg(+);Ad4(+/+) was larger than that of the wild type, and expectedly the size of the spleen of Tg(+);Ad4(-/-) apparently recovered.