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, I?B, MPF, PKA and MEK.

Leptomycin B is an unsaturated, branched-chain fatty acid, and is an important tool in the study of nuclear export. Leptomycin B is a specific inhibitor of proteins containing nuclear export signal. Leptomycin B inhibits nucleo-cytoplasmic translocation of molecules such as the HIV-1 Rev protein and Rev-dependent export of mRNA. The addition of very small amounts to fibroblasts causes accumulation of MEK in the nucleus. Other proteins that are influenced by leptomycin B are actin, c-Abl, cyclin B1, MDM2/p53, IκB, MPF, and PKA. The suggested inhibition mechanism involves the direct binding of leptomycin B to CRM1, which blocks the binding of CRM1 to proteins containing the nuclear export signal, via the interaction with cysteine residue in CRM1 control conserved region.