Neurotropic LCMV Armstrong is not selected for in the CNS of clone 13 infected mice. RNA was isolated from LCMV clones extracted from the brains of mice at 21 (n = 7 clones) and 150 days (n = 6 clones) post-clone 13 infection. RT-PCR, PCR and Mnl I restriction enzyme digests were performed as described in the Materials and Methods. The RNA PCR product from the Armstrong GP contains a phenylalanine at position 260 and is not cleaved by Mnl I. In contrast, clone 13 contains a leucine at position 260, and the 362 bp PCR product is cleaved into fragments (202 and 160 bp) by Mnl I. Note that all clones analyzed at both time points had the Mnl I restriction enzyme site. The control lane shows undigested 362 bp GP PCR product.