Finally, these studies also identify differences between M. dunni and FerrD2 cells that are clearly not receptor mediated. Use of LacZ pseudotypes shows that Spl574 Envs efficiently mediate entry into FerrD2 cells, but XC titers in Spl574 virus infected FerrD2 cells are clearly reduced as is virus-induced syncytium formation. This indicates a postentry block to virus replication leading to reduced surface Env, and the nature of this block is under investigation.