The human Y chromosome homologue of XG: transcription of a naturally truncated gene.

The XG blood group gene spans PABX1, the pseudoautosomal boundary on the X chromosome. The first three exons are pseudoautosomal and the remaining seven are X-specific. On the Y chromosome SRY and RPS4Y are located in Y-specific sequences within 70 kb of the boundary. Transcription from the XG promoter on the Y chromosome has been detected by cDNA cloning and PCR-based methods. Splicing of the pseudoautosomal exon 3 of XG occurs to multiple sites in Y-specific sequences. Transcripts detected include antisense SRY sequences and XG approximately RPS4Y hybrid transcripts. The heterogeneity and low abundance of transcripts as well as the lack of maintenance of the XG open reading frame in all but one transcript argue against a specific Y-chromosome gene product. An expressed pseudogene of XG, XGPY, has been mapped to interval Yq11.21. XGPY is transcribed and subject to alternative splicing. Sequence comparison suggests that XGPY originated from XG by a gene duplication event in the primate lineage.