Figure 1. In vitro splicing activity of nuclear extracts. Splicing was performed in various nuclear extracts (N10, N25, N26, N27) for 90 min in the presence of increasing concentrations of nuclear extract (0, 10, 20, 30, 40, and 50%, v/v). After the reactions, the RNA was recovered and analyzed on a 14% polyacrylamide/8 M urea gel, running in 0.5× TBE and visualized by autoradiography. The various RNA species associated with splicing in vitro are indicated schematically at the right. Quantification of the splicing efficiency of the N25 extract showed that approximately 25% of the input pre‐mRNA was converted to mRNA. The pre‐mRNA, labeled with [α‐]‐UTP, contains 58 U nucleotides, 37 of which reside in the intron.