Purity >98%. Leptomycin B is a potent inhibitor of the nuclear export of proteins. It directly blocks the interaction of CRM1 (exportin 1) with nuclear export signals (NES), preventing the export of a broad range of proteins. Leptomycin B will also indirectly inhibit the export of mRNA from the nucleus by blocking the nuclear export of mRNA-binding proteins (export factors). By blocking the nuclear export of p53, leptomycin B prevents entry of human normal fibroblasts into the S phase of the cell cycle, thus inhibiting proliferation. Leptomycin B was originally identified as an antifungal agent and is isolated from Streptomyces sp..