We previously determined that amino acids 64 to 120 of human T-cell lymphotropic virus type 1 (HTLV-1) Rex can restore the function of an effector domain mutant of human immunodeficiency virus type 1 (HIV-1) Rev (T. J. Hope, B. L. Bond, D. McDonald, N. P. Klein, and T. G. Parslow, J. Virol. 65:6001-6007, 1991). In this report, we (i) identify and characterize a position-independent 17-amino-acid region of HTLV-1 Rex that fully complements HIV-1 Rev effector domain mutants and (ii) show that this 17-amino-acid region and specific hydrophobic substitutions can serve as nuclear export signals. Mutagenesis studies revealed that four leucines within the minimal region were essential for function. Alignment of the minimal Rex region with the HIV-1 Rev effector domain suggested that the position of some of the conserved leucines is flexible. We found two of the leucines could each occupy one of two positions within the context of the full-length HTLV-1 Rex protein and maintain function. The idea of flexibility within the Rex effector domain was confirmed and extended by identifying functional substitutions by screening a library of effector domain mutants in which the two regions of flexibility were randomized. Secondly, the functional roles of the minimal Rex effector domain and hydrophobic substitutions were independently confirmed by demonstrating that these effector domains could serve as nuclear export signals when conjugated with bovine serum albumin. Nuclear export of the wild-type Rex conjugates was temperature dependent and sensitive to wheat germ agglutinin and was blocked by a 20-fold excess of unlabeled conjugates. Together, these studies reveal that position-variable hydrophobic interactions within the HTLV-1 Rex effector domain mediate nuclear export function.