Schematic diagram of the contiguous human SHBG/ABP transcript and alternatively spliced transcripts arising from the downstream promoter, P
. A structure/function diagram for the eight exon long SHBG/ABP transcript is shown above. The nucleotide positions of each exon are at the top (the exon 1 start codon defined as beginning with nucleotide +1); below that, the number of amino acids encoded by each exon. Conserved exons are numbered in white, alternatively spliced exons in yellow. Exon boundaries are denoted by horizontal white rectangles. Shown in red is the region of the SHBG/ABP transcript encoding the 29 amino acid long signal peptide, in light blue, the 383 amino acid long SHBG/ABP monomer. Steroid binding determinants in the mature SHBG protein are denoted by black rectangles, those forming the dimerization domain are blue, and that encoding the decapeptide sequence known to mediate binding of SHBG to RSHBG is purple. Below, in black, are linear depictions of the two previously described human SHBG gene transcripts originating from PL; the four novel transcripts described in this study are depicted in grey. Transcripts are identified on the left by missing exons. Three of five alternatively spliced PL transcripts retain the SHBG/ABP reading frame, whereas those lacking exon 7 alone and exons 4–7 undergo identical frameshifts within exon 8 that may affect their inherent stabilities (see discussion).