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It has been well known for some time that radiation can induce aberrations in the chromosomes of exposed cells. The frequency of occurrence of such aberrations increases with radiation dose, and certain kinds of aberrations persist throughout life, so the frequency of chromosome aberrations in human blood lymphocytes can be a useful indicator of the radiation dose received by the body. The cytogenetics program has studied chromosome aberrations in a subset of atomic-bomb survivors who participate in the Adult Health Study via several methods. Classical Giemsa staining allows one to visualize the chromosomes through a microscope and can detect about 2/3 of all chromosome aberrations. An extensive chromosome study using this traditional staining method was also conducted on about 16,000 children born to A-bomb survivors.

The cytogenetics laboratory now utilizes fluorescence in situ hybridization (FISH) to study the chromosomes of A-bomb survivors. In a multiple-step process that involves painting the chromosomes different colors, aberrations become apparent as hybrids of multi-colored chromosomes are detected. The FISH technique has enabled a more accurate and rapid identification of chromosomal aberrations than previous methods employed.

Although not a cytogenetics technique, electron spin resonance (ESR) in tooth enamel has been used as another indicator of radiation exposure. An ESR detector was installed in the cytogenetics laboratory in 1995 and has been used to examine several hundred tooth samples donated by A-bomb survivors. ESR is a powerful method for estimating individual radiation doses and the findings obtained to date corroborate the A-bomb survivors' chromosome aberration data.