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This is the first case of mosaicism unequivocally demonstrated in NBCCS. Furthermore, the mosaicism is unique in that the patient carries one normal and two mutant alleles. Because these mutations are located in close proximity, reversion error is likely to be involved in this event rather than a spontaneous mutation. In addition, this study indicates that gene analysis of iPSC clones can contribute to the detection of mosaicism containing a minor population carrying a second mutation.

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Noncoding regions within the cluster of immunoglobulin heavy chain constant genes in the human genome contained a number of repeats. In the mu-delta intron, two repeating units were contained. One 442-base-long fragment located JH-mu intron (defined as "sigma mu(sigma mu)") occupied the position in the mu-delta intron. The other 1166-base-long fragment located somewhere in front of S (class switch) region of C gamma gene was also found in the mu-delta intron. We defined the repeats in the mu-delta intron as "SIGMA (sigma)". The polarities of the longer repeats in the genome were opposite between the mu-delta intron and the upstreams of C gamma genes. These inverted copies (defined as sigma gamma 3 and sigma gamma 4), located 6 kb upstream of their respective C gamma's, were apparently transcribed in vitro, via RNA polymerase III and transcripts should have contained tRNA-like structures. Small DNA fragments capable of encoding tRNA-like structures were also found in corresponding regions of mouse Ig C gamma cluster.