Ran is an abundant member of the Ras superfamily of small GTPases that is highly conserved across eukaryotes. Originally cloned from a human teratocarcinoma cell line as one of four novel genes with sequence homology to the GTP-binding domain of Ras (Drivas et al. 1990), the gene was initially named TC4 (teratocarcinoma clone 4) and found to encode a protein of 216 amino acid residues. TC4 was markedly different from other members of the Ras superfamily in two respects: it lacked the sites required for post-translational lipid modification and it was primarily localized to the nucleus. Accordingly, it was renamed Ran for Ras-related nuclear protein. Ran was subsequently purified as an essential cofactor for nuclear protein import (Moore and Blobel 1993) and over the following years was extensively characterized for its role in regulating nucleocytoplasmic transport. Ran was later discovered to be critical for mitotic spindle assembly...