Detection of small amounts of human DNA in human-rodent hybrids

A method of measuring semi-quantitatively small amounts of human DNA in irradiated human x mouse and irradiated human x Chinese-hamster somatic cell hybrids is described. One method uses molecular hybridization of cell DNA bound to nitrocellulose filters with a cRNA probe to Cot 0--1 human DNA. Alternatively hybrid cell DNA is reassociated in solution with a Cot 0--1 fraction of nick-translated human DNA. Formamide buffers give specificity to the reaction. The detection limit of the filter method is 0.2--0.5% equivalents and reassociation kinetics 0.005--0.1% equivalents of a human genome. Experiments with cell hybrids suggest that a fragment of repetitive DNA may be retained along with the selected genes after a cell fusion. In one case however, of a hybrid cell in which malignancy is suppressed, highly repetitious sequences were not found.