The tRNA(Gly)/Glycyl-tRNA synthetase system belongs to the so called 'class II' in which tRNA identity elements consist of relative few and simple motifs, as compared to `class I' where the tRNA determinants are more complicated and spread over different parts of the tRNA, mostly including the anticodon. The determinants from `class II' although, are located in the aminoacyl stem and sometimes include the discriminator base. There exist predominant structure differences for the Glycyl-tRNA-synthetases and for the tRNA(Gly) identity elements comparing eucaryotic/archaebacterial and eubacterial systems. We Focus on comparative X-ray structure analysis of tRNA(Gly) acceptor stem microhelices from different organisms. Here, we report the X-ray structure of the human tRNA(Gly) microhelix isoacceptor G9990 at 1.18 angstrom resolution. Superposition experiments to another human tRNA(Gly) microhelix and a detailed comparison of the RNA hydration patterns show a great number of water molecules with identical positions in both RNAs. This is the first Structure Comparison of hydration layers from two isoacceptor tRNA microhelices with a naturally Occurring base pair exchange.