γ-Irradiation reduces histone H4 gene expression while selectively modulating the expression of p21CIP1/WAF1 and p27KIP1. U2OS cells were exposed to 5 Gy irradiation at 24 h after transfection. At 4 or 16 h after irradiation, total RNA was extracted and purified from triplicate experiments. The relative mRNA expression of H4n, p21CIP1/WAF1, p27KIP1, p57KIP2 and CDK2 was calculated with the ΔΔCT method using GAPDH as an internal control. For GAPDH, the CT values were plotted and these values did not change appreciably upon irradiation.