||P.3 right column 2nd paragraph: "Early observations indicated that the frequency of purine analogue resistance was increased by treating
cells with mutagens prior to selection. The spontaneous frequency of 8-AG[r] (8-Azaguanine resistance) or 6-TG[r] (6-thioguanine resistance) has been determined for a number of different cell types. While the precise frequency varies with the cell
line, density, mutant expression time (Chu et al., 1969 O’Neill et al., 1977) and analogue type and
concentration (Gillin et al., 1972 Sharp et al., 1973), it is reported to range from 10^-4 - 10^-6. In some studies, more careful analysis determined the frequency of spontaneous resistance to occur
at 1.5×10^-8 per cell per generation."