To define the relationship between syncytium formation and productive virus infection, transfected cell lines carrying either mCAT-1 or dCAT-1 were tested for susceptibility to a panel of ecotropic MLVs using the XC plaque overlay test (Table 1 ). In this assay, clusters of infected cells expressing ecotropic Env glycoprotein are identified by plaques of syncytia formed by overlaid rat XC cells [9] . For the cytopathic viruses Spl574 and F-S MLV, the number of syncytia induced directly by these viruses in susceptible cells is approximately equivalent to the titer determined by this XC overlay assay; for example, parallel cultures of infected M. dunni cells produced an XC titer of 10 5.1 (Table  1 ) compared to Spl574 syncytium titer of 10 4.6 .