The double-stranded RNA-specific editase 1 (RED1/ADAR2) is implicated in the editing of precursor-mRNAs (pre-mRNA) encoding subunits of glutamate receptors (GluRs) in brain. Site-specific deamination of adenosine to inosine alters the codon at the Q/R site in GluR-B rendering the heteromeric receptor impermeable to Ca2+ ions. We cloned human RED1 (hRED1/hADAR2) cDNAs from a brain cDNA library. The human enzyme is 95% identical to the rat homologue. We characterized two alternatively spliced forms that differed by the presence of an Alu-J cassette in the deaminase domain. For the long form containing the Alu cassette, we isolated cDNA clones with an alternative C-terminus and 3'-UTR. An 8.8-kb transcript of hRED1 is most abundant in brain and heart, and lower levels are detected in other tissues. In vitro editing assays with purified recombinant hRED1 containing or lacking the Alu-J cassette revealed that both forms of the protein have the same substrate specificity, but differ in their catalytic activity.