A restricted region within the TPO gene affects heterologous gene expression. L929 cells were transfected with 3 μg of pWTP-luc or pMTP-luc, respectively. Luciferase assay was performed as described in Materials and Methods. Luciferase activity of pWTP-luc was assigned to a value of 1.0, and the results are shown as the mean ± SD. The analysis was performed in duplicate assays and the results were reproducible.