The HOX11 gene was isolated from the chromosomal breakpoint of human T cell acute lymphoblastic leukemias with a chromosomal translocation t(10;14). Expression of this proto-oncogene is strictly controlled in normal tissues. However, regulatory elements of the gene have never been studied. Since the HOX11 gene is well conserved between human and murine, we sequenced 5' flanking region of the murine Hox11 gene and analyzed the elements. We identified the transcription start site (+1) of the gene using mRNA from fetal spleens by primer extension analysis. The start site was determined at 795 bp upstream from the ATG site. A typical TATA box sequence was found at 35 bp upstream from the start site. Furthermore, promoter activity of the 5' flanking region of the start site was monitored by luciferase assay. The activity mainly located within a 540-bp fragment immediately upstream from the start site (-540 to +1). The (-1240 to -540) region contained a negative regulatory element of the transcription. The TATA box sequence and the nucleotide sequence around the transcription start site were conserved in the human HOX11 gene. The transcription start site of the human HOX11 gene in normal tissues is discussed.