Binding of human RBCs to COS cells transfected with siglec-5 or CD33 cDNAs. Three days after transfection, COS cells were treated with sialidase or left untreated. Human RBCs, untreated or sialidase-treated, were added and allowed to bind for 30 minutes. After washing, cells were fixed and the number of rosettes (binding >2 RBCs) was determined by microscopy from counts of 20 fields in each well. Results show mean ± range of counts from duplicate wells and are representative of three experiments performed.