The Human Genome Contains Seven Genes for the β-Subunit of Chorionic Gonadotropin but Only One Gene for the β-Subunit of Luteinizing Hormone

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Abstract
We have used restriction enzyme analysis to sample similarities and differences among eight cloned genes, isolated from two independent libraries, which hybridize to a cDNA for the beta subunit of human chorionic gonadotropin (hCG). We find that one of the cloned genes encodes the β-subunit of the closely related hormone, human luteinizing hormone (hLH). Using differential hybridization of human DNA with β hCG- and β hLH-specific probes, we show that there are seven β hCG genes but only a single β hLH gene in the human genome. This single β hLH gene is linked to at least three β hCG genes. Our restriction analysis reveals that the seven β hCG genes are extremely similar but not identical, and that there are no obvious structural rearrangements to indicate which, if any, are pseudogenes. These data also reveal that the β hLH gene is far more homologous to the seven β hCG genes than the β hCG:β hLH protein sequence homology of 82% might predict. We propose that the β hCG gene arose by a duplication of an ancestral β hLH gene and that, once the β hCG function was established, the ancestral β hCG gene itself duplicated and rearranged, creating the present organization of multiple β hCG genes.