['It has been proposed previously that the ratio of complete to incomplete translocations as seen by fluorescence in situ hybridization (FISH), the S value, can be a cytogenetic fingerprint of exposure to radiation of different qualities. Results from a previous study suggested that the S value is approximately 10 for sparsely ionizing radiations such as X- and gamma-rays, and 2 for densely ionizing radiations. Based on FISH data of atomic-bomb (A-bomb) survivors, which showed an S value of 3.25, a significant neutron component to A-bomb radiation was suggested. To examine the possibility, the present in vitro study was conducted using X-rays.', 'Human blood lymphocytes were exposed to X-rays and first metaphases were examined with FISH using DNA probes for chromosomes 1, 2 and 4.', 'The S value was 3.16 for X-rays, which differs from approximately 10 as reported previously, and not larger than the 3.25 obtained from the blood lymphocytes of A-bomb survivors.'] Are s values a signature for a significant contribution of neutrons to the radiation dose received by atomic-bomb survivors?