HPRT1 and lymphoma: Many in vitro mutagenicity assays were developed throughout this period, including the ad-3 forward-mutation assay in N. crassa (de Serres and Kølmark 1958); cytogenetic assays (Kihlman 1966); Hprt (hypoxanthine-guanine phosphoribosyltransferase) assays in V79 (Chu and Malling 1968) and CHO cells (Hsie et al. 1975); the Tk+/− (thymidine kinase) assay in mouse lymphoma cells (Clive et al. 1972); and assays in yeast (Zimmermann 1971).