RANGAP1 and neoplasm: To further test if CRM1 is required for the cytoplasmic distribution of RanGAP1, we used another well-established approach to inhibit CRM1-mediated export by treating cells with a highly potent and specific inhibitor of CRM1 called leptomycin B (LMB), an antibiotic with anti-fungal and anti-tumor activity [32, 33].