Rex escorts unspliced gag-pro-pol and singly spliced env mRNAs out of the nucleus of infected cells. These mRNAs carry a recognition sequence called Rex responsive element (RxRE or XRE) located at the 3' region of the long terminal repeat (LTR). This function is essential since most HTLV proteins are translated from unspliced or partially spliced pre-mRNAs that cannot exit the nucleus by the pathway used by fully processed cellular mRNAs (By similarity). Homomultimer. The presence of both nuclear import (NLS) and nuclear export (NES) signals leads to continuous shuttling between the nucleus and cytoplasm. The RNA-binding motif binds to the RxRE, a complex secondary structure consisting of four stem loops and a long stretch of stem structure, present in incompletely spliced viral pre-mRNAs. This region also contains the NLS which mediates nuclear localization. These overlapping functions prevent Rex bound to RxRE from undesirable return to the nucleus. When Rex binds the RxRE, the NLS becomes masked while the NES remains accessible (By similarity). Belongs to the deltaretrovirus Rex protein family.