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. Isoform 1 is a metastasis suppressor with proapoptotic as well as antiangiogenic properties. Isoform 2 has an antiapoptotic effect. 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. Isoform 2 binds the proteasome subunit PSMD4/s5a through its N-terminus. (Microbial infection) Interacts with HIV-1 Tat (via activation domain). Ubiquitous. Highest level in liver. High levels in lung, skeletal muscle, pancreas and placenta. Moderate levels in heart and kidney. Low levels in brain. Not expressed or low levels in variant small cell lung carcinomas, 33% of hepatocellular carcinomas and neuroblastomas. Unique C-terminus confers high proteasome-dependent instability to isoform 2. Mutagenesis of Leu-154 and Leu-157 or Cys-158, Cys-160 and Cys-161 abolishes antiapoptotic effect. Was originally (PubMed:9482853 and PubMed:10698937) thought to be a transcriptional coregulator with protein kinase activity. However, crystal structure reveals a short chain dehydrogenase/reductase fold binding NADPH rather than ATP.