Staphylococcal nuclease R (SNaseR) is an analogue of Staphylococcal nuclease (SNaseA) . SNR141 is one of the N-terminal fragments of SNaseR, which has been highly expressed in and purified from E. coli and used to study nascent peptide folding. Single crystals of SNR141 suitable for X-ray analysis were grown in space group P41 with unit cell of a = b = 48. 2A and c = 63 . 8A by hanging drops vapour diffusion. Through the difference Fourier technique and restrained least square method, the crystal structure of SNR141 has been determined to 1 . 9A resolution with a final R factor of 0 .198 . Ther.m.s. deviations in the bond lengths and in the bond angels are 0 .016A and 3 .3°, respectively. The results of the crystal structure indicated that the deletion of 8 amino-acid residues from the C-terminal end of SNaseR perturbs the conformation of residue Tyr113 and blocks the binding between enzyme and substrate, which causes the biological activity of SNR141 down to 80% of wild type nuclease.