Promoter alignments of mouse and human GP Ibα gene sequences. Mouse (Mo) and human (Hu) GP Ibα gene sequences are aligned flanking the transcription initiation site of the human GP Ibα gene (nucleotide +1). Negative nucleotide numbering relative to the transcription initiation site is shown to the left of each sequence. The human GP Ibα gene is composed of a 5′ untranslated exon (exon I), a single intron, and a single exon (exon II) containing the initiating methionine codon (ATG). The exons are highlighted by a shaded box with only a limited 5′ portion of exon II displayed. Mouse exon I corresponds to nucleotides 2,387 to 2,466 and exon II begins at nucleotide 2,659 of GenBank accession no.U91967. The human 5′ sequence contains GATA and Etscis-acting elements, which have previously been shown by mutagenesis to be essential for promoter activity in megakaryocytic-like cell lines.20 Mutated bases of the human sequence that eliminated promoter activity are highlighted by black boxes at nucleotides −150 to −142 (Ets) and −92 to −91 (GATA).20 The mouse GP Ibα sequence displays a similar overall arrangement with conserved GATA and Ets elements along with positive regulatory element (MegPos) identified in the rat and human platelet factor 4 promoters.41 Translated sequence for the first 14 residues of the human and mouse GP Ibα signal peptides is shown in exon II with a single-letter notation for each residue except where there exists sequence differences, in which case both species-specific amino acids are shown. A BamHI restriction site is underlined (nucleotides 207-212) and corresponds to the 3′ boundary of the promoter fragment used to generate transgenic mice expressing the reporter protein, luciferase. Human GP Ibα DNA sequence corresponds to GenBank accession no. M22403.