Screening for Chemical Mutagens Using the Mouse Lymphoma Assay

The mouse lymphoma assay (MLA) using the thymidine kinase (Tk1 ) gene is widely used to detect the potential genotoxicity of a wide variety of chemical agents. The assay is recommended as a part of the core battery of genetic toxicology tests. Although the assay was developed more than 30 yr ago, there have been some technical modifications and also an evolution of the guidelines for the proper conduct and interpretation of data from the assay. In this chapter, the authors provide the details for the conduct of both the agar and microwell versions of the assay. They include their strategy for test chemical concentration selection and also provide a summary of the recommendations of an expert MLA Workgroup of the International Workshop for Genotoxicity Testing (IWGT). This group has been working over the past few years to discuss and reach consensus on a number of important aspects of the assay. Issues addressed include the appropriate measure for cytotoxicity, 24-h treatment, acceptance criteria for valid assays, and a new strategy for interpreting data.