Several approaches have been used to identify the factors involved in mRNA splicing. None of them, however, comprises a straightforward reversible method for inhibiting the second step of splicing using an external reagent other than a chelator. This investigation demonstrates that the addition of boric acid to an in vitro pre-mRNA splicing reaction causes a dose-dependent reversible inhibition e¡ect on the second step of splicing. The mechanism of action does not involve chelation of several metal ions; hindrance of 3P P splice-site; or binding to hSlu7. This study presents a novel method for speci¢c reversible inhibition of the second step of pre-mRNA splicing. ß