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A genomic cosmid clone for human sex hormone binding globulin (SHBG), a liver-secreted plasma glycoprotein that binds sex steroids, was isolated with a previously characterized liver cDNA as probe. Southern blot analysis of genomic DNA indicated that only one SHBG gene is present in the human haploid genome. A 3.8 Kb Xba I-fragment of the clone containing the entire coding region of SHBG was sequenced. The SHBG gene has 8 exons. The 5'-end preceding the translation start site had no TATA box or CAAT box promoter elements. Screening of a human testis cDNA library resulted in the isolation of two distinct cDNA forms. One cDNA was identical with the previously characterized liver SHBG cDNA, thus suggesting that human SHBG and the androgen binding protein (ABP) produced by Sertoli cells are coded for by the same gene. The second cDNA differed from the first by having exon I exchanged with a completely different sequence and exon VII deleted. An exon coding for the 5'-end of this cDNA was found in the cosmid clone 1.5 kb upstream of the first SHBG exon. Primer extension experiments showed the alternatively spliced transcript corresponding to the second cDNA to be present in both liver and testis. From the primary structure of this putative SHBG-gene-related protein, it may be deduced that it is a protein very different from SHBG and probably without steroid binding activity.