Five microliters of anti-digoxigenin-coated polystyrene beads (0.3 nM; diam. 0.49 mm) were mixed with 1 ml of the DNA-RNA hybrid ($1 mM) in binding buffer (10 mM Tris buffer [pH 7.0], 250 mM NaCl, 10 mM MgCl 2 , 0.4% w/v BSA), and incubated at 4 C for overnight on a rotator.