Leptomycin B is an anti-fungal antibiotic, anti-tumor cytotoxin that inhibits CRM1-dependent, NES-dependent nucleo-cytoplasmic translocation (nuclear export inhibitor). NES containing proteins include HIV-1 REV; actin, c-Abl, cyclin B1, MDM2/p53, IB, MPF, PKA and MEK.Leptomycin B is a potent and specific nuclear export inhibitor. Leptomycin B alkylates and inhibits CRM1 (chromosomal region maintenance)/exportin 1, a protein required for nuclear export of proteins containing a nuclear export sequence (NES). In addition to antifungal and antibacterial activities, Leptomycin B blocks the cell cycle and is a potent anti-tumor agent. At low nM concentrations, Leptomycin B blocks the nuclear export of many proteins including HIV-1 Rev, MAPK/ERK, and NF-κB/IκB, and it stabilizes the expression of p53. Leptomycin B also inhibits the export and translation of many RNAs, including COX-2 and c-Fos mRNAs, by inhibiting export of ribonucleoproteins. Leptomycin B is an anti-fungal antibiotic, anti-tumor cytotoxin that inhibits CRM1-dependent, NES-dependent nucleo-cytoplasmic translocation (nuclear export inhibitor). NES containing proteins include HIV-1 REV; actin, c-Abl, cyclin B1, MDM2/p53, IB, MPF, PKA and MEK. Leptomycin B is a potent and specific nuclear export inhibitor. Leptomycin B alkylates and inhibits CRM1 (chromosomal region maintenance)/exportin 1, a protein required for nuclear export of proteins containing a nuclear export sequence (NES). In addition to antifungal and antibacterial activities, Leptomycin B blocks the cell cycle and is a potent anti-tumor agent. At low nM concentrations, Leptomycin B blocks the nuclear export of many proteins including HIV-1 Rev, MAPK/ERK, and NF-κB/IκB, and it stabilizes the expression of p53. Leptomycin B also inhibits the export and translation of many RNAs, including COX-2 and c-Fos mRNAs, by inhibiting export of ribonucleoproteins.