A "housekeeping" gene on the X chromosome encodes a protein similar to ubiquitin.

An X chromosome gene located 40 kilobases downstream from the G6PD gene, at Xq28, was isolated and sequenced. This gene, which we named GdX, spans about 3.5 kilobases of genomic DNA. GdX is a single-copy gene, is conserved in evolution, and has the features of a "housekeeping" gene. At its 5' end, a cluster of CpG dinucleotides is methylated on the inactive X chromosome and unmethylated on the active X chromosome. The GdX gene can code for a 157 amino acid protein, GdX. Residues 1-74 of GdX show 43% identity to ubiquitin, a highly conserved 76 amino acid protein. The COOH-terminal moiety of GdX is characterized in its central part (residues 110-128) by a sequence homologous to the COOH-terminal hormonogenic site of thyroglobulin. The structural organization of the GdX protein suggests the existence of a family of genes, in addition to the ubiquitin gene, that could play specific roles in key cellular processes, possibly through protein-protein recognition.