Previous studies have suggested that pre-incubation of FIV with soluble Fc-CD134 fusion proteins may facilitate infection of CXCR4-expressing (CD134-negative) cells[50,51], the studies indicating that engagement of soluble CD134 triggers a conformational change in the FIV Env that exposes the CXCR4 binding site[50,51]. This evidence concerns the gene ERVW-1 and infection.