It was mentioned that the number of PCNA-positive cells was (i) in normal mice, 32.5 ± 2.9; (ii) in normal mice with arsenic exposure, 16.5 ± 1.4; (iii) in the chemically-induced depression lot, 12 ± 2.6; and (iv) in the lot of mice with chemically-induced depression and arsenic exposure, 3.6 ± 2.05. The gene discussed is PCNA; the disease is major depressive disorder.