Erratum
In the article "Splicing of messenger RNA precursors" by Phillip A. Sharp (13 Feb. p. 766), the results of C. Weissmann (53) were incorrectly described. As H. Hornig et al. show [Natur (London) 324, 589 (1987)], a C at the branch site of a precursor RNA does not arrest splicing at the intermediate stage, while either a U or a G at this position does arrest splicing at this point.