THE Ran proteins constitute a distinct branch of the superfamily of Ras-related GTP-binding proteins(1) which function as molecular switches cycling between GTP-bound ’on’ and GDP-bound ’off’ states(2). Ran is located predominantly in the nucleus of eukaryotic cells(3) and is involved in the nuclear import of proteins(4,5) as well as in control of DNA synthesis and of cell-cycle progression(6-8). We report here the crystal structure at 2.3 Angstrom resolution of human Ran (M(r) 24K) complexed with GDP and Mg2+. This structure reveals a similarity with the Ras core (G-domain) but with significant variations in regions involved in GDP and Mg2+ coordination (switch I and switch II regions in Ras)(9,10), suggesting that there could be major conformational changes upon GTP binding. In addition to the G-domain, an extended chain and an alpha-helix were identified at the carboxy terminus. The amino-terminal (amino-acid residues (1)MAAQGEP(7)) stretch and the acidic tail ((211)DEDDDL(216)) appear to be flexible in the crystal structure.