Recent study which was focused on the relationship between Δ2‐troglitazone (Δ2‐TGZ) and apoptosis in breast cancer cell discovered that cell apoptosis rate increased when ATF3 was highly going up.21 The similar result was revealed in our research that apoptosis rates both increased in MCF7 and SUM159 cells treated with radiation but overexpression ATF3 could slow it down. The gene discussed is ATF3; the disease is breast cancer.