The two intervening sequences of human beta- and gamma-globin pre-mRNAs are excised in a preferred temporal order in vitro.

Splicing of complete synthetic human beta- and gamma-globin mRNA precursors (pre-mRNAs) follows a preferred pathway in vitro. In almost all molecules, excision of the first intervening sequence (IVS) is completed before excision of the second IVS initiates. However, in a very small fraction of molecules the order of IVS excision is reversed.