Mode of CCR5-use. A. Schematic figure of CCR5 and the chimeric receptors FC-1, FC-2 and FC-4b. The CCR5 part is represented as grey and the CXCR4 part as black. B. Mode of CCR5-use was analyzed by infection of U87.CD4 cells expressing CCR5 or chimeric receptors. Length of bars indicates degree of infection and syncytia induction as observed in a light microscope 5 and 7 days after infection. Degree of infection follows a scale from 0 to 5 where 0 is no syncytia and RT negative; 1 was <10 syncytia per well or RT positive; 2 was 10–20 syncytia per well; 3 indicates syncytia covering 20–50% of the wells; 4 indicates syncytia covering 50–90% of the wells; 5 indicates syncytia covering >90% of the wells. RT production was positive in grades ranging from 2 to 5 and in concordance with syncytia induction.