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The sodium silt of guanylyl-3',5'-cytidine crystallizes in a monoclinic unit cell with one molecule in the asymmetric unit. Each molecule is related to another molecule by a 2-fold rotation axis which results in the formation of an antiparallel; right-handed .double helix with complementary hydrogen bonding between the guanine and cytosine residues. The crystal is heavily hydrated with 36 water molecules in the unit cell. The geometry of this crystalline double helix is. very similar to those which have been derived from studies of fiber x-ray diffraction patterns of double-stranded RNA; even though the latter do not yield data at atomic resolution.