A glyceraldehyde-3-phosphate dehydrogenase pseudogene on the short arm of the human X chromosomes defines a multigene family.

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Abstract
A human X chromosome‐derived gene sequence which recognises an abundant, 1.2‐kb mRNA in several cell types was previously isolated during a study to identify expressed sequences from an X chromosome recombinant library. Further characterisation of this clone, acronym OA1, has shown that it maps to the short arm of the X, at Xp21 to Xp22. A 777‐bp fragment of the clone which hybridizes to the 1.2‐kb mRNA has been sequenced, and the inferred amino acid sequence shows 80% homology with the published protein sequence for human muscle glyceraldehyde‐3‐phosphate dehydrogenase (GAPDH). The fragment shows even higher homology (87%) with pig muscle GAPDH. The OA1 clone selects an mRNA which translates in vitro into a polypeptide of 36 K, the subunit size of GAPDH. However, the X‐sequence is most probably a pseudogene whose structure is consistent with it having arisen by reverse transcription of a GAPDH or GAPDH‐related mRNA followed by insertion into the X chromosome. The GAPDH‐related portion of OA1 hybridizes to several DNA fragments in human and mouse DNA, and six fibroblast cDNA clones which cross‐hybridize to OA1 identify the same genomic fragments as OA1. This series of clones identifies a new, conserved GAPDH‐related multigene family.