Chemokines are a large family of secreted proteins that regulate the trafficking and activation of leukocytes by binding to G-protein-coupled receptors. In vitro studies of cloned and native receptors have shown that most chemokines bind to more than one receptor, and most chemokine receptors bind more than one chemokine. This suggests that the chemokine network may be a highly redundant system (1 ,2 ). Gene targeting has proven to be a useful tool for testing this hypothesis. So far the results suggest that each molecule has a specific biological role In vivo (3 –12 ).