Many mammalian cell-based gene mutation assays are available, but only four cell lines of Chinese hamster V97 and CHO cells, human lymphoblastoid TK6 cells, and mouse lymphoma L5178Y cells, and only three genetic loci of HPRT (hypoxanthine-guanine phosphoribosyltransferase), TK (thymidine kinase) and the cell membrane Na+/K+ ATPase gene, are well validated and widely used [30]. Here, HPRT1 is linked to lymphoma.