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The human X chromosome has a unique biology that was shaped by its evolution as the sex chromosome shared by males and females. We have determined 99.3% of the euchromatic sequence of the X chromosome. Our analysis illustrates the autosomal origin of the mammalian sex chromosomes, the stepwise process that led to the progressive loss of recombination between X and Y, and the extent of subsequent degradation of the Y chromosome. LINE1 repeat elements cover one-third of the X chromosome, with a distribution that is consistent with their proposed role as way stations in the process of X-chromosome inactivation. We found 1,098 genes in the sequence, of which 99 encode proteins expressed in testis and in various tumour types. A disproportionately high number of mendelian diseases are documented for the X chromosome. Of this number, 168 have been explained by mutations in 113 X-linked genes, which in many cases were characterized with the aid of the DNA sequence.

Original publication




Journal article



Publication Date





325 - 337


Animals, Antigens, Neoplasm, Centromere, Chromosomes, Human, X, Chromosomes, Human, Y, Contig Mapping, Crossing Over, Genetic, Dosage Compensation, Genetic, Evolution, Molecular, Female, Genetic Linkage, Genetics, Medical, Genomics, Humans, Male, Polymorphism, Single Nucleotide, RNA, Repetitive Sequences, Nucleic Acid, Sequence Analysis, DNA, Sequence Homology, Nucleic Acid, Testis