[en] Gamma radiation, as a quantitative perturbation reference, has been related to oxygen toxicity as the unavoidable background risk due to living in an oxygen atmosphere. The basic mechanisms shared by gamma irradiation and oxygen toxicity have been studied. The response to these two perturbations has been characterized at the molecular level through DNA chemistry and monoclonal antibodies, and by cellular biological responses. The investigation of cellular responses is being extended to the molecular level through a study of alteration of gene arrangement and gene expression. Concentration has been on the study of the involvement of the evolutionally conserved repetitive DNA sequences shared by hamster and man. Such sequences were found and some have been isolated in plasmids. Two cellular systems were chosen for investigation, the embryonic/adult mesenchymal system and the hematopoietic tissues system. Concentration has been on the isolation, properties, and response to perturbation of the progenitor cells and the stem cell populations