Figure 1. Distinct export pathways from the nucleus. CRM1 and NXF1 represent two key export pathways from the nucleus. Studies of retroviruses and retroelements have been critical for their discovery. CRM1 is essential for the Rev-mediated HIV mRNA export and expression. NXF1 is the molecular link between the constitutive transport element (CTE)– and RNA transport element (RTE)–containing mRNAs and the nuclear pore complex. The key factors mediating mRNA export of retrovirus and retroelement mRNA export also promote transport of cellular mRNAs and proteins. Rev, HIV-1 protein essential for the production of structural proteins and infectious virions; RRE, Rev-responsive element; SRV, simian type D retrovirus.