Parsimonious kinetic models of Rev export and different mechanisms of CRS/INS function. A) A typical cellular pre-mRNA, with typical splicing sites, even with a heterologous RRE, is not exported by Rev. B) Introduction of a "retention" or "aberrant splicing" CRS/INS into a mRNA retards splicing, allowing Rev/RRE export to effectively compete. C) Instability factors inducing degradation might be proposed to interact directly with CRS/INS, but stability alone without retardation of splicing would still not allow Rev/RRE export the opportunity to compete. D) Similarly, were Rev/RRE merely to release such an instability factor, rapid splicing would still outcompete Rev/RRE export, as in A. SD, splice donor; SA, splice acceptor; E#, exon#.