Purity >99% by HPLC. A potent, specific inhibitor of nuclear export signal (NES)-dependent protein export from the nucleus. It alkylates and inhibits CRM1 (chromosomal region maintenance)/exportin 1(XPO1), a protein required for nuclear export of proteins containing a nuclear export sequence . At low nM concentrations, Leptomycin B blocks the nuclear export of many proteins including HIV-1 Rev, MAPK/ERK, and NF- Kappa B/I Kappa B, and it stabilizes the expression of p53.