CD8A and infection: The following important functional domains are also identical: the putative cell surface receptor binding sites and neutralizing domains on the surface gp51 env protein; the peptide region in gp51 that induces a CD8+ cytotoxic T-cell response in the host cow; the highly immunogenic epitope GD21 that is conserved in all members of the PTLV/BLV genus; the tetrapeptide WAPE (aa 222–225 in Fig. 4) that has been shown to be critical for infection; and the amino acids P and D (aa 210 and 211 in Fig. 4) that have been shown to induce T-helper proliferative responses in cattle [15-17].