Importin proteins mediate regulated nucleocytoplasmic transport by selectively binding cargo and transporting them through the nuclear pores.  Many are essential for normal development, including gametogenesis. Our studies of importins (IMPs) in the context of spermatogenesis have revealed that they probably have a much broader functional capacity than previously appreciated.  We documented the regulated synthesis of individual importin mRNAs and proteins, both importin alphas (IMP aand betas (IMPb, at distinct stages of spermatogenesis, and we propose that this serves to coordinate appropriate transport of particular transcription factors into the nucleus to generate sperm. This led to the surprising observation that, although they are predominantly cytoplasmic, certain IMPs are predominantly, if not exclusively, nuclear-localized in meiotic and haploid male germ cells. Through several approaches, we identified IMPa-specificbinding partners that differ between spermatocytes and round spermatids. Remarkably, many of these are not nuclear proteins, and several are essential for male fertility.  IMP proteins and certain of their binding partners localize to distinct regions in mature sperm, highlighting a new potential role for IMPs to facilitate protein trafficking to non-nuclear sites.  In cell lines, such as HeLa cells. IMPas become sequestered in the nucleus following transient exposure to cellular stressors, such as hydrogen peroxide.  We learned that nuclear-localized importins can control transcription of a relatively small number of genes, with STK35 identified as one of the two that are upregulated.  STK35 is highly expressed in the testis, particularly in the germ cell subtypes that contain nuclear-localized IMPa proteins. implicating importins in germ cell stress responses. Analysis of mouse strains with higher and lower levels of IMPa4 in spermatids provided the first evidence that nuclear IMPa protein can spermatid viability in response to oxidative stress.  Thus IMPα functions during spermatogenesis may include regulating transcription factor access to the nucleus, directly influence transcription within the nucleus and directing subcellular localization of proteins required for male fertility.