Oxidoreductase required for tumor suppression. NADPH-bound form inhibits nuclear import by competing with nuclear import substrates for binding to a subset of nuclear transport receptors. May act as a redox sensor linked to transcription through regulation of nuclear import. Monomer. Binds nuclear transport receptors XPO4, IPO5/RANBP5, IPO7, IPO9 and KPNB1 as well as GCN1L1/GCN1 and LRPPRC probably through their HEAT repeats. Binds NCOA5/CIA (By similarity). Mice are haploinsufficient for tumor suppression. 50% develop tumors within their second year. 30% of the tumors are hepatocellular carcinomas.