 Research carried out in the 1980s revealed that a Chinese hamster ovary (CHO) cell mutant called XR-1 was "extremely sensitive" with regard to being killed by gamma rays during the G1 portion of the cell cycle but, in the same research studies, showed "nearly normal resistance" to gamma-ray damage during the late S phase; and in the course of this research, XR-1's cell-cycle sensitivity was correlated with its inability to repair DNA double-strand breaks produced by ionizing radiation and restriction enzymes.In particular, in a study using somatic cell hybrids of XR-1 cells and human fibroblasts, Giaccia "et al.