Induction of genomic DNA-nicking in apoptotic PLB-985 cells treated with 10⁻⁶ mol/L 9-cis-RA for 3 days. Cytospins were prepared from (A) normal and (B) RA-treated cells, and the TUNEL assay was performed, labeling the apoptosis-specific DNA strand breaks with fluorescent label as described in Materials and Methods.