CXCR4 and infection: Our previous studies showed that Laboratory-adapted strains (TCA) of FIV can bind to HSPG through a HSPG binding region that involves both the N-terminal and the C-terminal sides of the V3 loop, thus facilitating productive infection of adherent cell lines (HSPG++, CXCR4+, CD134-) such as CrFK and G355-5 that lack expression of the normal primary binding receptor, CD134 [35].