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.[1] Leptomycin B will also indirectly inhibit the export of mRNA from the nucleus by blocking the nuclear export of mRNA-binding proteins (export factors).[2] 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.[3]

Reference:
[1]. Kudo, N., Wolff, B., Sekimoto, T., et al. Leptomycin B inhibition of signal-mediated nuclear export by direct binding to CRM1. Exp. Cell Res. 242(2), 540-547 (1998).
[2]. Nishi, K., Yoshida, M., Fujiwara, D., et al. Leptomycin B targets a regulatory cascade of crm1, a fission yeast nuclear protein, involved in control of higher order chromosome structure and gene expression. The Journal of Biological Chemisty 269(9), 6320-6324 (1994).
[3]. Smart, P., Lane, E.B., Lane, D.P., et al. Effects on normal fibroblasts and neuroblastoma cells of the activation of the p53 response by the nuclear export inhibitor leptomycin B. Oncogene 18, 7378-7386 (1999).