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A new member of the human RNase A superfamily is reported. Identified in the human genome assembly as LOC 390443, this locus is located 128 kb telomenc to the established RNase A gene family cluster on chromosome 14q 11.2. The amino acid sequence of this locus is sufficiently similar to the eight previously identified gene family members to wanant a designation as RNase 9. RNase 9 is expressed in a wide range of human tissues. In addition, a 3D-amino acid sequence lying between a 26-amino acid putative signal peptide and the last 148 amino acids that align with the other RNases A is not seen in other members of the RNase A supelfamily in any species. Nucleotide and amino acid sequences of RNase 9 in 13 nonhuman primate species were detennined and indicate several conserved sites but, also, an excess of nonsynonymous substitutions, about one-third of which are radical substitutions. This suggests that RNase 9, similar to several other human RNases A, has been under diversifying selection in the primates. Data from the mouse and rat genomes indicate that RNase 9 is also present in rodents, thus making it older than most of the established members of the human RNase A superfamily. Many of the human RNases A have been shown to have antimicrobial, antiviral, or antiparasitic functions involved in host-defense mechanisms. The features of RNase 9 described here suggest that it, too, may be involved in host defense and that it, along wilh the rest of the superfamily, may prove to have played an important role in anthropoid evolution.