.... Table L Summary of cytogcnctic ExPosure group (rads) 175 70 cells scored Subjccts (No.) (No,) 30 13 1500 650 43 2150 findinbs in Marshall Isltmders exposed to fallout radiation. Pcrcenmges appcw in parcnihcses. Chromosome aberrations Cells wit h Sub- 46 2n + (%) jects as%eled 10 8 Frag- Dicen- ments tncs 12 11 11(0.73) 10(1.53) 23 21 (0.93) 6(0.4) 2(0.3) Totals for 8 “ 400 9 5.5 who were exposed to fallout radiation (9). . These people were accidentally exposed to radioactive fallout on Rongelap Island in 1954: the wind shifted unpredictably after the detonation of a high-y ield’nuclear device at Bikini, 160 km away; 64 inhabitants were exposed to 175 rads ( estimate~ whole-body) of gamma rays. Eighteen other Rongelap Islanders, away fishing on a neighboring island, were exposed to 70 rads (estimated). Exposure resulted in temporary depression of blood cells, skin bums from beta radiation, and internal absorption of radionuclides, the most important being iodine and strontium. It was estimated that the thyroid glands of the adults received 150 rads—those of the children, as much as 1,000 rads —from absorbed radioiodine in addition to the whole-body exposure to gamma rays. Recent development of pathological changes in the thyroid is believed to have resulted from thts ex. posure (10). Chromosome preparations were obtained from 5 i individuals during the annual medical review of the Marshall kl~nders in 1964; 30 of them had been exposed to 175 rads; 13, to 70 rads of gamma rays. Eight Marshall Islrmders who had escaped exposure were in. eluded for comparison. , The exposed group comprised 20 males and 23 females, their ages ranging from embryo irt u~ero to 67 years at the time of exposure. Twenty-one were under 20 and 22 over, with males and females about equal in number. The comparison group included three males and five females ranging in age from 10 to 71 years; Unfortunately it was impossible to examine more unexposed individuals matched to the exposed with respect to age and sex. Cultures of peripheral blood lymphocytes were made by use of a modification of the meti’od of Moorhead et al. (11 ); they were harvested at 48 and 446 5 TotaIs for ,9(2.25) exposed 8(0.37) unexposed Rings 1(0.15) Chroma- TotN chromoTransio- cations Total tid aberra- some tions (No.) breaks 5(0.33) 8(1.23) 22(1.46) 21(3.23) 33(2.2) 32(4.9) 58(3.8) 35(s.3) 13(0.61) 43(2.0) 65(3.02) 93(4.3 ) 9(:.25) 11(2.7) subjects 1(0.04) subiects 72 hours (12). The slides were stained with aceto-orcein and examined by phase microscopy. Because comparison of “the 48- and 72-hour cultures from showed no differences five individuals in aberration rates, all examinations but one were made on 50 cells of each individual from the 72-hour cultures. Al. together 2150 cells were examined from the 43 exposed subjects; 400 cells from the eight unexposed. Aberrations were scored as follows (i) aneuploidy, including polyploid cells, (ii) chromosome aberrations, and (iii)’ chromatid aberrations. Karyotypes were made in cases” where the counts were equivocal or where chromosomes of questionable morphology were seen. Only aberrations that were agreed on by several observers were included in the final tabulation. Additional karyotypes were made from eight euploid cells, showing ‘no gross abnormalities, from each of four subjects showing more aberrations than most others; this was done in an effort to detect minor structural abnormalities such as small deletions or inversions that could easily escape notice under the microscope. l%al]y, bone-marrow preparations were made from two exposed subjects and one control; only one preparation from an exposed subject contained enough suitable cells in metaphase to warrant evacuation. Proportionally the largest number of aberration-positive persons appeared in the low-exposure group (Table 1); we cannot explain this paradox and so treat all the exposed subjects as one group in comparing them with theltnexposed (Table 1). Chromosome-type aberrations were found in 23 of the 43 exposed peopie and in five of the eight unexposed. The high incidence of acentric fragments in the latter group was unusual and unexpected, and we cannot account for it. The total numbers of all types of chromosome aberrations in the two groups 9(2.25) were 43 and 9—rates of 2.0 and T percent, respectively. Thus superfic~ there appears to be no difference tween the two groups, but we sh point out that in the exposed gJ half of alI chromosome aberrations sisted of two-break aberrations SUC, dicentric chromosomes, translocat: and a ring, the remainder being ace: fragments. In contrast, no two-b aberrations were found in the L, posed group in which all aberr:t were acentric fragments. Only sevc the exposed people resembled the trois in showing single-break a/tions onl~ all others showed two-’ aberrations either alone or in coc, tion with fragments. The clifferenc tween the exposed and unex, groups in the relative incidence oi break aberrations and acentric ments was significant at the 1-pi level (P< .01). Aberrations ranged from one t., per person. Sixteen of the 23 esl and two of the five unexposed pc had multiple aberrations. - In m~ stances no more than one aber~ ‘ was noted per cell. Aneuploidy higher in the exposed than in the ~ posed group, but the difference not significant; in most cells, arteu; was due to loss of chromosomes number of chromatid and isochrm gaps and breaks, although som higher in the exposed group, was in normal limits. Polyploid ccli ctsrred with equaI frequency in th groups. No aberrations were fou the karyotypes that had been from the 32 grossly intact cells the four subjects showing relative[? rates of aberration. Likewise, n. normalities were seen in 50 cells the bone marrow of one of the es. subjects. We pointed out earlier that th portion of aberration-positive p in the low-exposure group wa~ that among the highly exposed. \\ SCIENCE, V{