Human oligoadenylate synthetase 1 (OAS1) recognizes double-stranded RNA (dsRNA) typically produced by viral infections. The X-ray crystal structure of human OAS1 bound to a model 18-bp dsRNA duplex revealed that dsRNA binding allosterically drives a functionally essential structural reorganization within human OAS1 that narrows the adenosine triphosphate (ATP)-binding cleft and repositions a catalytic residue to complete its active site (Donovan J et al. 2013). Once stimulated by dsRNA, OAS1 uses ATP to synthesize a series of 5'-triphosphorylated 2'-5'-linked oligoadenylates containing 2 to greater than 5 adenylyl residues. The 5'-triphosphorylated 2'-5’-linked triadenylate (i.e., ppp5’A2’p5’A2’p5’A) is typically the most abundant active species (Kerr IM & Brown RE 1978).