Rho proteins have been implicated in a variety of cytoskeletal functions, but it is unclear how Rho proteins regulate these cellular functions and how Rho proteins are regulated. In this study, we cloned the rhoB gene from the mouse genome and characterized its promoter region. The predicted amino acid sequence was identical to that encoded by the human rhoB gene. A site for initiation of transcription was found at position -376 relative to the site for initiation of translation. Deletion analysis of the 5'-flanking region of the rhoB gene revealed that the minimum region of the promoter was located between positions -507 and -376. Northern blotting analysis showed that the expression of the mouse rhoB gene was induced by serum, suggesting that expression of the rhoB gene might be controlled by some signal-responsive element(s).