Noncoding regions within the cluster of immunoglobulin heavy chain constant genes in the human genome contained a number of repeats. In the μ-δ intron, two repeating units were contained. One 442-base-long fragment located JH-μ intron ( defined as "sigma μ(σμ)") occupied the position in the μ-σ intron. The other 1166-base-long fragment located somewhere in front of S (class switch) region of Cγ gene was also found in the μ-δ intron. We defined the repeats in the μ-δ intron as "SIGMA (Σ)". The polarities of the longer repeats in the genome were opposite between the μ-δ intron and the upstreams of Cγ genes. These inverted copies (defined as σγ3 and σγ4), located 6 kb upstream of their respective Cγ'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 Cγ cluster.
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