HIF (hypoxia inducible factor) family members play a central role in cellular and systemic oxygen homeostasis. HIF-1 is the prototypical member of the family and consists of two subunits, HIF-1a and HIF-1b. The dimerization of the subunits take place in the nucleus and is necessary for DNA binding and transcriptional activation of target genes. While the different factors concerned with regulation and stability of HIF-1a are described in more detail, the knowledge of the translocation process is still imprecise. Nuclear import occurs via the nuclear pore complex (NPC). Therefore, passing the NPC is one of the major events in a HIF-1 dependent transactivation process. We provided strong evidence that nuclear import of HIF-1a is a highly regulated process mediated by various a importins. So far, six human a importins have been described. cDNAs encoding GST-tagged versions of all ubiquitously expressed human aimportins as well as importin b were constructed and recombinantly expressed. NLS-mutants of HIF-1a were generated by site-directed mutagenesis. GST-tagged importins were purified, incubated with in vitro translated variants of HIF-1a, and GST pull-down experiments were performed. Import efficency was shown by Immunofluorescence analysis. In this study we mapped primary structure elements essential for HIF-1a-Importin a interaction.