The study investigates the formation of novel histone H2AX species, referred to as γ-H2AX, in mammalian cell cultures and mice exposed to ionizing radiation. γ-H2AX is identified as a form of H2AX that is specifically phosphorylated at serine 139. The formation of γ-H2AX is rapid and sensitive to DNA double-stranded breaks, occurring within seconds after exposure to ionizing radiation and reaching a maximum within 10 minutes. Approximately 1% of H2AX becomes γ-phosphorylated per gray of ionizing radiation, indicating that about 0.03% of the chromatin is involved per DNA double-stranded break. This suggests that large amounts of chromatin are involved in each DNA double-stranded break, providing insights into higher-order chromatin structures. The study also demonstrates that γ-H2AX formation is not dependent on DNA-PK, suggesting the involvement of an alternative DNA double-stranded break repair system.The study investigates the formation of novel histone H2AX species, referred to as γ-H2AX, in mammalian cell cultures and mice exposed to ionizing radiation. γ-H2AX is identified as a form of H2AX that is specifically phosphorylated at serine 139. The formation of γ-H2AX is rapid and sensitive to DNA double-stranded breaks, occurring within seconds after exposure to ionizing radiation and reaching a maximum within 10 minutes. Approximately 1% of H2AX becomes γ-phosphorylated per gray of ionizing radiation, indicating that about 0.03% of the chromatin is involved per DNA double-stranded break. This suggests that large amounts of chromatin are involved in each DNA double-stranded break, providing insights into higher-order chromatin structures. The study also demonstrates that γ-H2AX formation is not dependent on DNA-PK, suggesting the involvement of an alternative DNA double-stranded break repair system.