C10H13N5O4, Mr = 267.2, monoclinic, P2(1), a = 5.716 (3), b = 11.998 (8), c = 17.658 (10) A, beta = 94.26 (4) degrees, V = 1208 A3, Z = 4, D chi = 1.47 g cm-3, lambda (Mo K alpha) = 0.71069 A, mu = 7.5 cm-1, F(000) = 560, T = 293 K. R = 0.060 for 2138 unique observed [F greater than 4 sigma (F)] reflections. The N-glycosidic torsion angles chi have values -125.9 (5) and -172.0 (5) degrees, in the anti range. (Molecule-A values are given first throughout.) The sugar puckers are 2(3)T (C3'-exo/C2'-endo), with P = 171 (1) degrees and psi m = 14 (1) degrees, and 4(3)T (C4'-endo/C3'-exo), with P = 213 (1) degrees and psi m = 11 (1) degrees. The C4-C5 conformations, with gamma = 49.7 (5) and 173.7 (5) degrees, are + sc (gauche-gauche) and ap (gauche-trans). The conformational parameters used follow the guidelines of the IUPAC-IUB Joint Commission on Biochemical Nomenclature [Pure Appl. Chem. (1983), 55, 1273-1280]. The molecules in the asymmetric unit form a hydrogen-bonded, base-paired dimer. The bonding is as follows: N3A-0.973 A-H3A... 1.790 A...O2B, N3A...O2B 2.747 (8) A, angle at H3A 167 degrees and N3B-0.992 A-H3B...1.916 A...O2A, N3B...O2A 2.894 (8) A, angle at H3B 168 degrees. The propeller twist between the bases is 5 degrees [Wilson & Tollin (1987). Nucleosides Nucleotides, 6, 643-653].