35 25).71 No two-hit aberrations were found in the unexposed group, but both groups had an unusual numberof acentric fragments, the cause of which is not known. Paradoxically, Rongelap people with the lower exposure had more aberrations than those with the higher exposure. These studies indicate that a small but significant numberof chromosomeaberrations persisted in blood lymphocytes in some Marshallese as late as 10 years after exposure. The results are consistent with those of similar studies on the exposed Japanese fishermen,’? on victims of other radiation acci- dents,’3 and on Japanese bombsurvivors. ’# 2. Somatic Mutations In 1974, studies* were made of the frequency of amino acid substitution in the hemoglobin of the Marshallese on blood samples sent to Oak Ridge.75 Since there is no coded isoleucine”® in adult human hemoglobin A,its presence must be due to errors in transcription or translation or to somatic mutations arising during DNAreplication. Errors in transcription, which occur infrequently, form altered mRNA, tRNA, and rRNA; they change the coding in the mRNA and may reduce fidelity of the tRNA with regard to beth thé id” of amino acid it accepts and the mRNA codonsit recognizes.?? Errors in translation”? arise through the attachment of wrong amino acids to tRNA (aminoacyl synthetase errors) and the imprecise recognition of mRNAcodons by tRNAanticodons (translational variation). Somatic mutations result from mistakes in replication of DNA; manysinglebase-substitution mutations change nonisoleucine into isoleucine codons,’® and the resulting mutant cells could have hemoglobin mRNAwith isoleucine codons. For this reason, an increasein theisoFigure 25. Two-hit chromosome aberrations in exposed Marshallese. Top: arrow points to dicentric form; bottom: arrow points to ring form.!? leucine content of hemoglobin A would be expected in humans exposed to agents causing basesubstitution mutations. Possibly radiation may cause base-substitution mutations in human somatic cells, but this has never been established. > 20) and 8 unexposed Rongelap people.* Chromosome aberrations were noted in 23 of the exposed and in 5 of the unexposed Marshallese, but the exposed group had a numberof two-break aberrations (represented by dicentric chromosomes, translocations, and a ring form) that are thought to be associated with radiation exposure (Figure “These analyses were done by Drs. H. Lisco, New England Deaconess Hospital, and R.A. Conard, BNL. Theisoleucine content of the hemoglobin A was determined in blood from 13 exposed Marshallese and 12 unexposed. The frequency of isoleucine substitution for other aminoacids in hemoglobin was calculated by dividing the nanomolesofisoleucine by the total nanomoles ofall other amino acids in each sample. Thefrequencies arelisted in Table 21. * These studies were done by Drs. R.A. Popp, G. P. Hirsch, and E.G. Basliff at Oak Ridge National Laboratory.