The mouse nodal gene is expressed during early embryogenesis and plays an important role during gastrulation and the determination of the left-right body symmetry/axis formation. The embryonic stem (ES) cell line 413-d 3563 contains a single copy of a retrovirus insertion in the first inton of the nodal gene. Using a quantitative RNase protection assay, the level of the nodal gene expression in this cell line was compared to that of the parental CCE cell line. A 2.3 fold down regulation of the nodal transcript in the 3563 ES cell line suggested that the proviral insertion might decrease the nodal mRNA expression by disrupting the potential cis regulatory elements present at the insertion site in the first intron of the nodal gene. Luciferase reporter constructs containing the entire first intron fragment or part of it were tested by a transient transfection assay for their potential transcriptional regulatory activity. A 2.4 kb DNA fragment from the 5' end of the intron was sufficient to increase the expression level of the reporter gene 8 fold, suggesting that an important cis acting enhancer element for the nodal gene expression is localized within the first intron.