The study investigates the formation and distribution of γ-H2AX foci in mammalian cells exposed to ionizing radiation, which is a critical threat to chromosomal integrity. γ-H2AX, a modified form of histone H2AX, is rapidly phosphorylated at serine 139 upon DNA double-strand breaks, forming discrete nuclear foci. The antibody against γ-H2AX detects these foci in various species, including humans, Xenopus laevis, Drosophila melanogaster, and Saccharomyces cerevisiae. These foci appear within 1 minute after exposure to ionizing radiation and are comparable in number to the induced DNA double-strand breaks. In mitotic cells, band-like structures of γ-H2AX foci form on chromosome arms, indicating that γ-H2AX forms en masse at chromosomal sites of DNA double-strand breaks. The study suggests the existence of higher-order chromatin structures involved in monitoring DNA integrity. The findings provide direct visual confirmation of the immediate and substantial response of cells to DNA double-strand breaks, highlighting the conservation of this response across evolution.The study investigates the formation and distribution of γ-H2AX foci in mammalian cells exposed to ionizing radiation, which is a critical threat to chromosomal integrity. γ-H2AX, a modified form of histone H2AX, is rapidly phosphorylated at serine 139 upon DNA double-strand breaks, forming discrete nuclear foci. The antibody against γ-H2AX detects these foci in various species, including humans, Xenopus laevis, Drosophila melanogaster, and Saccharomyces cerevisiae. These foci appear within 1 minute after exposure to ionizing radiation and are comparable in number to the induced DNA double-strand breaks. In mitotic cells, band-like structures of γ-H2AX foci form on chromosome arms, indicating that γ-H2AX forms en masse at chromosomal sites of DNA double-strand breaks. The study suggests the existence of higher-order chromatin structures involved in monitoring DNA integrity. The findings provide direct visual confirmation of the immediate and substantial response of cells to DNA double-strand breaks, highlighting the conservation of this response across evolution.