Table 1.

HPRT mutation rates

Cell line	Affected MMR locus*	Biochemical deficiency	Mutation frequency†	HPRT mutation rate mutations/generation‡
HL-60	—	—	9.6 ± 3.5 × 10−8	4.7 ± 1.4 × 10−8
HL-60R§	MSH3 amplified	hMutSα	2.2 ± 0.9 × 10−4 (+ Mtx)	2.6 ± 1.2 × 10−5 (+ Mtx)
			4.4 × 10−4 (no Mtx)	5.0 × 10−5 (no Mtx)
HeLa	—	—	1.4 × 10−8	1.0 × 10−8
LoVo	MSH2 deletion	hMutSα, hMutSβ	3.7 × 10−5	4.9 × 10−6

Mutation rates were determined as described in Materials and Methods. 

^*Mismatch repair (MMR) defect. 

^†Mutation frequency is expressed as HPRT mutants per colony forming unit (CFU) ± 1 SD. Total CFUs from which 6-thioguanine mutants were selected ranged from 2-10 × 10⁷ cells. 

^‡± One standard deviation. 

^§HL-60R cells were cultured in the presence or absence of 1 μM Mtx as indicated.