Two sublines of strain RFM mice with different radiation sensitivity characteristics were used to investigate the genetic involvement of a radiation resistance factor. Resistant and nonresistant sublines, their F1 cross and the F1 backcross to each parent subline were exposed to low-level continuous γ-rays, and radioresistance characteristics were determined by mean survival time. The F1 backcross was H-2 tested, and mice were tested for radioresistance according to genotype <tex-math>${\rm H}\text{-}2^{{\rm ff}}$</tex-math> or <tex-math>${\rm H}\text{-}2^{{\rm fk}}$</tex-math>. The data are consistent with the hypothesis that this radioresistance factor is controlled at a single gene locus with resistance dominant to sensitivity. However, the H-2 gene did not prove to be directly responsible for the difference in radiation resistance between the two lines.

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