Hypothetical mechanism of altered nucleocytoplasmic traffic of wild-type and mutant NPM. (Left) NPM wild-type (wt) cells show nucleus-restricted NPM positivity. Nuclear export of NPM wild-type (NPMwt) is minimum since it contains only one NES motif available for binding to CRM1 (Exportin 1). Tryptophans 288 and 290 drive NPMwt to nucleolus (Nc), thus acting as a counterforce to NES-dependent NPMwt nuclear export. As a consequence, no NPMwt is detectable in the cytoplasm by immunohistochemistry. (Right) In cells carrying NPM exon 12 mutations, NPM mutants accumulate in the cytoplasm (NPMc⁺), since they contain 2 NES motifs (1 created at C-terminus by the mutation) available for CRM1 binding. Moreover, they carry mutated tryptophans 288 and 290, which inhibit binding of NPM leukemic mutants to nucleolus (Nc), facilitating their nuclear export. A portion of NPmwt (which escapes NPM mutant recruitment) can bind to nucleoli (not shown). The circled 1 indicates where leptomycin B blocks nuclear export of NPM leukemic mutants by specific inhibition of CRM1. Red squares represent physiologic NES motif; gray squares, new C-terminus NES motif; black ovals, tryptophans 288 and 290; and orange ovals, mutated tryptophans 288 and 290.