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The gene, located at Xq28, which encodes factor 8, the antihemophilic factor (q.v.). HEMA contains 189 kilobases of DNA and is one of the largest human genes. It has 26 exons and 25 introns. One of these (number 22) is gigantic (32 kb), and it contains a CpG island (q.v.). This serves as a bidirectional promoter for two genes (A and B). Gene A contains no introns and is transcribed in the opposite direction from factor 8. Gene B is transcribed in the same direction as factor 8. Its first exon is in intron 22, and this exon is spliced to exons 23–26 to form the complete transcript. The A and B transcripts are found in many different tissues, but the functions of the proteins they encode are not known. Two additional copies of gene A also lie about 500 kilobases upstream of HEMA. Intrachromosomal recombination between the A gene within intron 22 and either of these duplicates can yield an inversion that disrupts HEMA. Such inversions are thought to cause 25% of all cases of hemophilia A. The gene is expressed in the liver. The first human mutations caused by transposable elements were discovered in HEMA. See Chronology, 1988, Kazazian et al.; hemophilia.

Subjects: Genetics and Genomics.

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