17, but with the following changes: (a) use 25% of the reaction volume, i.e., about 1.25 million nuclei/0.25 ml digestion; and (b) vary the DNase I concentration in each tube, for example, 0, 10, 20, 40, 80, and 120 U of DNase I per ml. Alternatively, the concentration of DNase I may be fixed, and a variable incubation time may be employed (e.g