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Spontaneously generated mutant T cells defective in T-cell receptor (TCR) gene expression are detectable at the frequency of 2×10-4 in vivo, and the mutant fractions are dose dependently increased by exposure to genotoxic agents such as ionizing radiation. Mutant cells with altered expression of TCRα or -β among CD4+ T cells can be detected as CD3-/CD4+ cells by two-color flow cytometry using anti-CD3 and anti-CD4 monoclonal antibodies labeled with different fluorescent dyes, because incomplete TCRαβ/CD3 complexes cannot be transported to the cellular membrane. This flow cytometric mutation assay can be applied to CD4+ T cells from human peripheral blood and mouse spleen. Methods for both preparation of target cells and detection of the mutant cells are described. [ABSTRACT FROM AUTHOR]