To explore further the mechanism for the reduced virulence of MR/PR(prM) in vivo when compared to MR766, IFNAR-/- mice were infected s.c. with 1 × 104 PFU of MR766, PRVABC59, MR/PR(C), MR/PR(prM) or MR/PR(E), and tissue virus titers determined. The gene discussed is IFNAR1; the disease is miotic rate.