After the wells were washed, the cells were fixed and the intracellular DNA was denatured using fixing/denature agent. Subsequently, the wells were washed, and the anti-BrdUPOD conjugate was added to each well for 1 h. After they were thoroughly washed, the samples were incubated using substrate solution for 20 min. After stop solution was added, the OD was measured at 450/550 nm using a microplate reader.