Radio-immunoassay (RIA) of UV-induced 6-4PP and CPD. Twelve hours after UVB irradiation, genomic DNA was isolated from treated cells and assayed for UV-induced DNA damage using lesion-specific antibodies. An average of fifty-five CPD and twelve 6-4PP lesions per million bases were detected after exposure to 300 J/m2 UVB. Results are the average of duplicate assays from two independent experiments. Error bars show standard deviation.