As a high level of antigenic stimulation is crucial for the development of broad and potent NAb responses (Rodriguez et al., 2007; Doria-Rose et al., 2009; Sather et al., 2009), it is possible that the immune systems of those cats that did not neutralize pseudotypes bearing autologous Env proteins had not been exposed to sufficient antigenic stimulation following infection to induce NAbs. This evidence concerns the gene ERVW-1 and infection.