Carboxylated fluorescent microspheres (Luminex, Austin, TX) were suspended in 80 l of 0.1 M NaH 2 PO 4 (pH 6.2) and activated by adding 10 l (each) of 50 mg/ml N-hydroxysuccinimide (Pierce, Rockford, IL) and 50 mg/ml 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (Pierce).