Since syncytium formation is based on the presence of viral envelope and viral receptors on the cellular surface, syncytia can be simply induced by gp160 expression. Therefore, HeLa P4 cells were transfected with gp160 expression vector pSVATGrev. Transfected HeLa P4 cells expose the gp120/gp41 complex on their outer membrane surface and syncytia could be detected after 28 h (Figure 5e ). In contrast to non-transfected cells, incubation with mHSA did not show any syncytia (Figure 5d ). 1000) to LAV-gp120 was tested by surface plasmon resonance spectroscopy (Biacore 1000, GE Healthcare). Purified LAV gp120 (10 μg/ml, Protein Sciences Corporation) was covalently linked using EDC/NHC to a dextran coated, CH-activated sensor chip (CM5, GE Healthcare). After blocking with ethanolamine the biosensor was washed three times with 30 μl 100 mM NaOH and 30 μl 100 mM HCl (flow rate 5 μl/min). Proteins were tested for binding at 1, 5, 10, 25 and 50 μg/ml at a flow rate of 5 μl/min. (g). Binding of mHSA, mBSA and mMSA (values marked by the black arrow in Figure 1a, 1c and 1e ) was compared to the binding of CD4, and gp120-specific antibodies to a LAV gp120 coated CM5 biosensor. ADP429: anti-serum, derived from HIV-1 SF2 gp120-immunized sheep (AIDS reagent project, NIBSC, UK). EVA3047: monoclonal antibody, derived from IRIQRGPGRAFTIGC-peptide immunized mice (AIDS reagent project, NIBSC, UK). Figure 5f , incubation of pSVATGrev transfected cells with mHSA (50 μg/ml) again totally inhibited syncytium formation.