The human X chromosome has a unique biology shaped by its evolution as a sex chromosome shared by males and females. The study determined 99.3% of the euchromatic sequence of the X chromosome, revealing the autosomal origin of mammalian sex chromosomes, the stepwise loss of recombination between X and Y, and the degradation of the Y chromosome. LINE1 elements cover one-third of the X chromosome, consistent with their role in X-chromosome inactivation. The sequence includes 1,098 genes, with 99 encoding testis- and tumour-specific proteins. A disproportionately high number of Mendelian diseases are linked to X-linked genes. The X chromosome has unique features, including X-chromosome inactivation in females, which equalizes X-linked gene expression between sexes. The X chromosome's evolution from autosomes led to the loss of genes from the Y chromosome and the hemizygosity of males for X-linked genes, contributing to the high prevalence of X-linked diseases. The X chromosome sequence was completed using PAC and BAC clones, with a high accuracy of over 99.99%. The sequence covers 155 Mb, including 16 contigs and 150 million base pairs. The sequence includes 1,098 genes, 700 pseudogenes, and non-coding RNA genes. The X chromosome has a low gene density and high repeat content, with a low (G+C) content. The sequence includes 173 non-coding RNA genes and 2 tRNA genes. The XIST gene is critical for X-chromosome inactivation. The X chromosome contains 99 cancer-testis antigen genes, including novel members of several families. The X chromosome has a high density of LINE1 elements, which may be involved in X-chromosome inactivation. The X chromosome sequence reveals the evolutionary history of the X chromosome, including the expansion of gene families and the loss of recombination between X and Y. The X chromosome has a unique structure, with the centromere containing higher-order repeat sequences. The X chromosome sequence includes 153,146 SNPs, with 901 non-synonymous changes. The X chromosome has a unique evolutionary history, with homology to chicken autosomes. The X chromosome has a large region of homology with the chicken genome, including the XAR and XCR. The X chromosome has a unique evolutionary history, with the loss of recombination between X and Y. The X chromosome has a unique structure, with the centromere containing higher-order repeat sequences. The X chromosome sequence includes 153,146 SNPs, with 901 non-synonymous changes. The X chromosome has a unique evolutionary history, with homology to chicken autosomes. The X chromosome has a large region of homology with the chicken genome, including the XAR and XCR.The human X chromosome has a unique biology shaped by its evolution as a sex chromosome shared by males and females. The study determined 99.3% of the euchromatic sequence of the X chromosome, revealing the autosomal origin of mammalian sex chromosomes, the stepwise loss of recombination between X and Y, and the degradation of the Y chromosome. LINE1 elements cover one-third of the X chromosome, consistent with their role in X-chromosome inactivation. The sequence includes 1,098 genes, with 99 encoding testis- and tumour-specific proteins. A disproportionately high number of Mendelian diseases are linked to X-linked genes. The X chromosome has unique features, including X-chromosome inactivation in females, which equalizes X-linked gene expression between sexes. The X chromosome's evolution from autosomes led to the loss of genes from the Y chromosome and the hemizygosity of males for X-linked genes, contributing to the high prevalence of X-linked diseases. The X chromosome sequence was completed using PAC and BAC clones, with a high accuracy of over 99.99%. The sequence covers 155 Mb, including 16 contigs and 150 million base pairs. The sequence includes 1,098 genes, 700 pseudogenes, and non-coding RNA genes. The X chromosome has a low gene density and high repeat content, with a low (G+C) content. The sequence includes 173 non-coding RNA genes and 2 tRNA genes. The XIST gene is critical for X-chromosome inactivation. The X chromosome contains 99 cancer-testis antigen genes, including novel members of several families. The X chromosome has a high density of LINE1 elements, which may be involved in X-chromosome inactivation. The X chromosome sequence reveals the evolutionary history of the X chromosome, including the expansion of gene families and the loss of recombination between X and Y. The X chromosome has a unique structure, with the centromere containing higher-order repeat sequences. The X chromosome sequence includes 153,146 SNPs, with 901 non-synonymous changes. The X chromosome has a unique evolutionary history, with homology to chicken autosomes. The X chromosome has a large region of homology with the chicken genome, including the XAR and XCR. The X chromosome has a unique evolutionary history, with the loss of recombination between X and Y. The X chromosome has a unique structure, with the centromere containing higher-order repeat sequences. The X chromosome sequence includes 153,146 SNPs, with 901 non-synonymous changes. The X chromosome has a unique evolutionary history, with homology to chicken autosomes. The X chromosome has a large region of homology with the chicken genome, including the XAR and XCR.