we have measured the forward mutation rate at the hypoxanthine phosphoribosyltransferase ( hprt ) gene of the human promyelocytic leukemia cell line hl-60 and have determined the molecular spectrum of spontaneous hprt mutations in 45 independent 6-thioguanine-resistant hl-60 sublines .
C1415701
hypoxanthine
C0020687
hprt