Table 2.

Spontaneous resistance to BrdU^R (TK⁻) and 6-TG (HPRT⁻) in HAM^sl, HAM⁺, and HAM⁻ strains

Phenotype	Resistance to
	BrdU	6-TG
K1 HAMsl		6.0 × 10−5	≈10−6*
HAM+	≈10−6†	<10−6*
HAM−	1.2 × 10−2	4.6 × 10−3

The values are rates per cell division determined by fluctuation tests (see Materials and Methods). 

^*6-TG^R colonies in K1 are mainly mutations (15) and a comparable number may occur in HAM⁺, but this was not determined. 

^†Of 10 populations (3.2 × 10⁵ viable cells), only 3 produced at least one colony, so the median cannot be estimated. The value given is the frequency (scoring each of the populations with BrdU^R colonies as one), which is higher than the median.