Processing is initiated once the SLBP (bound to the stem loop) and the U7 snRNP (bound to the HDE) are both loaded onto the pre-mRNA. The pre-mRNA HDE makes base-pairing contacts with the 5”² end of U7 snRNA. Binding of the U7 snRNP to the pre-mRNA is stabilized by interactions between a U7 snRNP protein, hZFP100 and SLBP. It should be noted that there must be other trans-acting factors, including the factor that catalyzes the cleavage reaction, which have yet to be defined. The cleavage occurs in the presence of EDTA as does the cleavage reaction in polyadenylation, it is likely that this reaction is catalyzed by a protein. There may well be additional proteins associated with the U7 snRNP, and since in some conditions in vitro processing occurs in the absence of SLBP, it is possible that all the other factors required for processing are associated with the active form of the U7 snRNP.