Regulated nucleocytoplasmic transport is of central importance to the fidelity of tightly regulated gene expression that is required for gametogenesis and adult fertility¹ .  Our work has identified many aspects of protein transport between the cytoplasm and nucleus that are modulated throughout spermatogenesis.  We are now working to delineate the physiological importance of key molecules involved in this, with a particular focus on the importins/karyopherins.  Importin α (IMPα) proteins reside predominantly in the cytoplasm and are classically understood to function with an importinβ1 molecule to ferry cargo containing a nuclear localisation signal (NLS) into the nucleus. Non-transport functions for IMPα proteins are emerging; we recently showed that IMPα proteins act within the nucleus to regulate gene transcription. The Serine threonine kinase 35 (Stk35) transcript was singularly upregulated in HeLa cells exposed to cellular stress conditions that provoked IMPα nuclear accumulation².  Remarkably, we had previously shown that IMPα4 is nuclear in spermatocytes and spermatids^3, cells in which STK35 is transcribed.  This presentation will describe discoveries from our three new mouse strains which indicate that IMPα proteins not only selectively deliver essential cargo to the nucleus, but also directly modify cell fate through nuclear actions including transcriptional regulation and driving nuclear remodelling events spermatogenesis.