The involvement of CAT-1 in the cytopathic response in M. dunni cells was suggested by the specific sequence differences that distinguish the dCAT-1 receptor variant from mCAT-1. These 2 receptors differ by 4 amino acids of which two are within the third extracellular CAT-1 loop that contains the virus binding site: I214V, and a glycine insertion within the YGE virus binding site [6] . As shown in the present paper, all cells expressing the dCAT-1 variant and none expressing mCAT-1 are susceptible to virusinduced syncytium formation. This indicates that one or both of these two amino acid changes, I214V and Δ236G, are responsible for the cytopathic response mediated by this receptor variant.