The introduction of single-cell tissue culture techniques by Puck and sociates (1) marked the advent of a new chapter in quantitative cellular rad with mammalian cells. These techniques were almost immediately applie one of the most important and perplexing problems in radiobiology-where and what are the lethal sites in a mammalian cell. In bacteria and yeasts, viability has been found to be ten to one hundred times more sensitive to X-rays than are important macromolecules, as measured by biochemical or physicochemical changes. This fact has been viewed as evidence supporting the connection between genic damage and viability although other cellular systems capable of amplifying small effects cannot be excluded on this basis alone.

Puck et al. have shown (2, 3) that mean lethal doses in mammalian cells are smaller still by another one to two orders of magnitude. This, plus the fact that the first cell line examined in detail, the HeLa line S3, displayed a" hitness" number of 2, led to the reasonable hypothesis that the sites sensitive to X-irradiation in mammalian cells reside in the chromosomes and that radiation" hits" are evidenced by chromosomal breaks (2). The postulated connection between a" two-hit" survival curve and lethality resulting from a two-hit aberration also implied that, at least in the HeLa cell, there was only one locus at which such events could occur. Subsequent studies with other cell lines, both of neoplastic and normal origin, did not support the simple" two-hit" hypothesis, since" hitness" numbers between 1 and about 3 were observed (3, 4). In an effort to resolve these inconsistencies, a generalized model for the inactivation of a cell was presented (5) which related survival to the combined influences of three inactivation schemes, thus permitting the model to be fitted to essentially all survival curves on the appropriate choice of a number of adjustable parameters.