Importin can exist as either a heterodimer of importin-α/β or as a monomer of Importin-β. Importin-α was first isolated in 1994 by a group including Enno Hartmann, based at the Max Delbrück Center for Molecular Medicine. The process of nuclear protein import had already been characterised in previous reviews, but the key proteins involved had not been elucidated up until that point. A 60 kDa cytosolic protein, essential for protein import into the nucleus, and with a 44% sequence identity to SRP1p, was purified from "Xenopus" eggs. It was cloned, sequenced and expressed in "E.coli" and in order to completely reconstitute signal dependent transport, had to be combined with Ran(TC4). Other key stimulatory factors were also found in the study.