Studies on functional compatibility of various Rev proteins derived fr
om all known human and simian immunodeficiency virus subgroups have sh
own that this essential gene product is not always exchangeable among
the viruses. In an attempt to map the region of Rev proteins responsib
le for the observed nonreciprocal complementation, hybrid genomic Rev
expression vectors were constructed by exchanging the first and second
exons of rev genes, and were examined for their abilities to activate
reporter clones by transfection. With one exception, the second codin
g exon of rev gene determined the functional specificity of Rev protei
ns.