Intron excision is an essential step in eukaryotic gene expression, but the molecular mechanisms by which the spliceosome accurately identifies splice sites in nuclear precursors to messenger RNAs (pre-mRNAs) are not well understood. A bimolecular assay for the second step of splicing has now revealed that exon ligation by the human spliceosome does not require covalent attachment of a 3’ splice site to the branch site. Furthermore, accurate definition of the 3’ splice site in this system is independent of either a covalently attached polypyrimidine tract or specific 3’ exon sequences. Rather, in this system 3’ splice site selection apparently occurs with a 5’ --> 3’ directionality.