TKT and lymphoma: Common Phase 1 in vitro mammalian tests include: the mouse lymphoma thymidine kinase (TK) gene mutation assay, which detects compounds that induce forward gene mutations in the tk gene of the L5178Y mouse lymphoma cell line, and the hypoxanthine guanine phosphorybosyl transferase (HPRT) gene mutation assay, which identifies agents that cause gene mutations in the hprt gene of a suitable cell line, such as Chinese hamster cells (Combes et al. 2007; Eastmond et al. 2009; Johnson 2012).