However, in the present study, which was conducted in vitro in the complete absence of antibodies or complement, we demonstrate that while a mean of only 2.3% of HIV-1 p24 bound to erythrocytes, this small amount of binding accounted for binding of up to 97% of the infectious virions, and the cell-bound virions were then approximately 100-fold more infectious, via trans infection, for infecting CD4(+) cells. This evidence concerns the gene CD4 and infection.