 Importin-β, unlike importin-α, has no direct homologues in yeast, but was purified as a 90-95kDa protein and found to form a heterodimer with importin-α in a number of different cases.These included a study led by Michael Rexach
 and further studies by Dirk Görlich.These groups found that importin-α requires another protein, importin-β to function, and that together they form a receptor for nuclear localization signals (NLS), thus allowing transport into the nucleus.