In vitro splicing of WT and EX-3 substrates. The WT and EX-3 mutation pre-mRNAs were synthesized and subjected to in vitro splicing reactions using HeLa nuclear extracts. The RNAs were resolved on a 4% polyacrylamide gel containing 8 M urea and visualized by autoradiography. The precursors, intermediates, and final products are indicated at left. The RNA molecular weight markers in nt are at right. The arrow indicates product with exon 16.