In our quest to determine what alterations in the genotype of sPV1(M) resulted in the observed neurophenotypic changes, two strategies were employed: (1) exchange of genomic segments between sPV1(M) and wt PV1(M) followed by analysis of neurovirulence in vivo; and (2) sequence analysis of viruses recovered from the spinal cords ofsPV1(M)-inoculated CD155 tg mice that had succumbed to infection in concert with comparison of these sequences with that of sPV1(M) virus that constituted the inoculum. This evidence concerns the gene PVR and infection.