H2AX phosphorylation as a genotoxicity endpoint.

The gammaH2AX focus assay, based on phosphorylation of the variant histone protein H2AX, was evaluated as a genotoxicity test in immortalised wild-type mouse embryonic fibroblasts (MEFs) treated for 4h with a panel of reference compounds routinely used in genotoxicity testing. The topoisomerase II poison etoposide (0.006-60 microg/ml), the alkylating agent methyl methanesulfonate (1.