Transfected M. dunni cells expressing mCAT-1 in addition to the endogenous dCAT-1 gene were significantly more susceptible to MoMLV infection than untransfected M. dunni cells (Table 1) , consistent with a previous study indicating that the dCAT-1 sequence variation is responsible for M. dunni resistance to MoMLV [6] . No difference was noted in the XC plaque titer of Spl574 in M. dunni cells expressing mCAT-1 in addition to the endogenous dCAT-1, and no viruses other than Spl574 and F-S MLV were cytopathic in the mCAT-1 transfected M. dunni cells.