Large-scale mapping of branchpoints in human pre-mRNA transcripts in vivo.

We present the first large-scale identification of lariats-the transient branched introns that are released as a byproduct of pre-mRNA splicing. The locations of the branchpoints in these introns provide insight into the early steps of splicing. From this data set, we have developed a comprehensive model of 3' splice-site selection, identified new mechanisms of alternative splicing and mapped the distribution of splicing factors around branchpoints.