Overlapping genes are relatively common in DNA and RNA viruses. There are several examples in bacterial and eukaryotic genomes, but, in general, overlapping genes are quite rare in organisms other than viruses. There have been a few reports of overlapping genes in mammalian genomes. The present study identified all of the overlapping loci and overlapping exons in every chromosome of the human genome using a public database. The total number of overlapping loci on the same and opposite strands was 949 and 743, respectively. Similarly, in every chromosome, the instances in which two loci were located on the same strand was similar to the number of 2 genes observed on opposite strands, except for chromosome 5. The number of 2 exons located on the same strand was higher than that for 2 exons located on opposite strands, indicating the presence of many comprehensive-type overlaps. The mean percentage of overlapping exons on opposite strands in each chromosome was 3.3%, suggesting that parts of the nucleotide sequences of 26,501 exons are used to produce 2 transcribed products from each strand. The ratio of the number of overlapping regions to chromosomal length revealed that, on chromosomes 22, 17 and 19, ratios were high for both types of 2 loci, with exons located on the same and opposite strands. Ratios were low on chromosomes Y, 13 and 18. These results show that all overlapping types are distributed throughout the human genome, but that distributions differ for each chromosome.
Keywords: chromosome; exon; human genome; locus; overlapping genes.