EraCode bead coupling For each conjugation reaction, a mixture (400 L) containing ϳ5 ϫ 10 6 carboxylated microspheres (Luminex Corporation) was centrifuged at 8000g for 1 min. The supernatant was removed, and the beads were resuspended in 50 L of 0.1 mol/L MES, pH 4.5. After resuspension, 3.3 L of 300 mol/L 5Ј-amine-modified EraCode DNA oligonucleotide and 5.0 L of freshly prepared 10 g/L EDC (Pierce Chemical) were added. The reaction was mixed thoroughly and incubated at room temperature in the dark for 30 min. A second 5.0 L of 10 g/L EDC was added, and the mixture was incubated for 30 min. Era-Code microspheres were washed by the addition of 1.0 mL of 10 mmol/L Tris (pH 8.0) containing 0.2 mL/L Tween 20 and pelleted by centrifugation at 8000g for 1 min; the supernatant was then removed. EraCode microspheres were washed a second time with 1 g/L sodium dodecyl sulfate in 10 mmol/L Tris (pH 8.0) and resuspended in 100 L of bead buffer (10 mmol/L MOPS, pH 7.5; 1 mmol/L EDTA, 0.5 g/L sodium dodecyl sulfate, 200 mmol/L sodium chloride, and 0.1 g/L sonicated herring sperm DNA). EraCoded microspheres were combined and diluted in bead buffer to generate a 50-code mixture at a concentration of 800 microspheres/L for each class of microsphere.