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The catalytic domain (amino acid residues 1 to 166) of the human ras-oncogene product p21 complexed with the GTP analogues beta,gamma-imido-GTP (GMPPNP), beta,gamma-methylene-GTP (GMPPCP), and guanosine-5'-(gamma-thiotriphosphate) (GTP gamma S) have been been crystallized. Crystals of the GMPPNP and GMPPCP complexes are well suited for high resolution X-ray crystallography. They belong to space group P3(1)21 (or its enantiomorph P3(2)21) with unit cell axes a=b=40.3 A and c = 162.2 A.