As shown in the previous section, radiation effects on chromosomes remain in lymphocytes even many years after radiation exposure, and reflect effects on genetic material contained in the chromosomal DNA. Therefore, radiation effects on genes resulting in mutations in “somatic cells” (all the cells of the body other than the reproductive cells of the ovary or testes) were also expected to remain in the body. Following tests of several assay methods using blood cells, it was found that they did not record dose effects except for mutation in the glycophorin A (GPA) gene in red blood cells. The frequency of GPA mutant cells, however, varied extensively among ordinary people who were not exposed to radiation, which prevented us from evaluating individual radiation dose from the frequency. This observation is understandable if we assume that, while the mutation induction rate is generally on the order of 10^-4 per Gy, the total number of longterm stem cells in bone marrow would not be sufficiently large, e.g., several 10⁵. Thus, the GPA test is not considered useful for individual dose evaluation but collective dose of a group of people with similar exposures.