Chromosome maintenance region 1 (CRM1), a protein that shares sequence similarities with the karyopherin beta family of proteins involved in nuclear import pathway, was shown to form a complex with the leucine-rich nuclear export signal (NES), This interaction was inhibited by leptomycin B, a drug that prevents the function of the CRM1 protein in yeast, To analyze the role of the CRM1-NES interaction in nuclear export, a transport assay based on semipermeabilized cells was developed. In this system, which reconstituted NES-, cytosol-, and energy-dependent nuclear export, leptomycin B specifically blocked export of NES-containing proteins, Thus, the CRM1 protein could act as a NES receptor involved in nuclear protein export.