Copy number abnormalities such as deletions and duplications give rise to a variety of medical problems and also manifest innocuous genomic variations. Aberrant DNA replication is suggested as the mechanism underlying de novo copy number abnormalities, but the precise details have remained unknown. In our present study, we analyzed the del(2)(q13q14.2) chromosomal junction site observed in a woman with a recurrent pregnancy loss. Microarray analyses allowed us to precisely demarcate a 2.8 Mb deletion in this case, which does not appear in the database of human genomic variations. This deletion includes only one brain-specific gene that could not be related to the reproduction failure of the patient. At the junction of the deletion, we found that 11-13-nucleotide sequence, originally located at the proximal breakpoint region, was repeated four times with a single-nucleotide microhomology at the joint between each repeat. The proximal region and the distal region was finally joined with six-nucleotide microhomology. The structure of the junction is consistent with backward replication slippage proposed previously. Our data lend support to the notion that a common DNA replication-mediated pathway generates copy number variation in the human genome.
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