Isolation and characterization of a DNA fragment containing various kinds of repetitive sequences located on human chromosome 21

In order to investigate the repetitive sequences located on human chromosome 21, we have isolated DNA fragments containing Alu sequences. One of the clones, p1, was chosen for further study, because it contained repetitive sequences different from the Alu sequence. Nucleotide sequence analysis of p1 indicates that p1 contains L1 and O-family sequences. Interestingly, when the L1 sequence was used as a probe, a discrete band of 5 kb was seen in HindIII-digested DNA from somatic cell hydbrids containing human chromosome 21 as the sole human chromosome. The L1 sequence was rearranged and was interrupted by O-family sequence, which wasd flanked by 6 bp target site dup litations. Since all three repetitive sequences are known to act as retroposons, these results imply that there is an integration hot spot on human chromosome 21. The sequence was mapped within 21q11–21.