Competition of ¹²⁵I–MIP-1α binding to HEK 293 cells transfected with CHEMR1, CCR-4, and CCR-1. Transfected cells (2 × 10⁵ cells) were incubated with 0.1 nmol/L ¹²⁵I–MIP-1α and varying concentrations of unlabeled MIP-1α at 37°C for 1 hour. The incubation mixtures were layered on 10% sucrose gradients, and labeled cells were recovered from pellets. In the case of CCR-1, 25% of the total input ¹²⁵I–MIP-1α (∼10⁵ cpm) bound to transfected cells in the absence of cold hMIP-1α, whereas the background level was less than 200 cpm.