CRM1 mediates the nuclear export of proteins exposing leucine-rich nuclear-export signals (NESs). Most NESs bind to CRM1 with relatively low affinity. Recently, higher-affinity NESs were selected from a 15-mer random peptide library. Unexpectedly, complexes between high-affinity NESs and CRM1 accumulate at the cytoplasmic filaments of the nuclear pore complex (NPC). This finding suggests that high-affinity NES binding to CRM1 impairs the efficient release of export complexes from the NPC, explaining why leucine-rich NESs have evolved to be weak.