(a-c) Post-mitochondrial supernatant fraction of mouse L cells was used to write the word “2-5A” on nitrocellulose filters. Subsequently, the filters were incubated in a solution of the 2-5A probe (a) without additions, (b) with unlabeled 2-5A as competitor, and (c) with unlabeled ATP. The filters were washed, dried and used to expose x-ray film. The results on autoradiography show that binding was prevented with unlabeled 2-5A but not with ATP, thus demonstrating the feasibility of the approach. The first clone for a partial murine RNase L was obtained by screening a cDNA expression library of IFN and cycloheximide treated mouse L cells. The third round screening was done (d) without competitor, (e) with unlabeled 2-5A as competitor, or (f) with unlabeled ATP.