We study the transforming ability of X-rays in multistep carcinogenesis by irradiating primary rat cells which contain transfected c-myc oncogene. X-irradiation induces fully transformed phenotypes, including anchorage-independent growth and tumor formation in nude mice. Of seven foci examined, five exhibited an A to G conversion in codon 61 of the N-ras oncogene. Another transformed isolate has a single G to A base substitution in codon 14 in the same oncogene, while no point mutation is detected in the other focus. This is the first in vitro demonstration of the association between point mutation and X-ray-transformed cells.

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Supported in part by Grant DE-FG02-89ER60903 from the Department of Energy.

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