A protein that is a translation product of the human CD34 gene or a 1:1 ortholog thereof. It is a leukocyte membrane protein expressed specifically by lymphohematopoietic progenitor cells. It contains a single-pass transmembrane domain and that show distinct expression on early hematopoietic precursors and vascular-associated tissue. Acts as a scaffold that presents selectin carbohydrate ligands in a clustered, tissue specific manner to allow for higher avidity interactions between leukocytes and endothelial cells during the inflammatory process. In common with related sialomucins (endoglycan and podocalyxin), the extracellular region is dominated by an N-terminal mucin-like domain, which is densely substituted with sialylated O-linked carbohydrates. The mucin-like region is followed by a cysteine-containing and presumably globular domain. This domain may fold into an immunoglobulin-like structure as the positions of 2 of the cysteines are conserved in the C2 set of the immunoglobulin superfamily. The cytoplasmic domain is around 73-76 residues long and highly conserved. [ http://www.ncbi.nlm.nih.gov/pubmed/8983065 http://www.ncbi.nlm.nih.gov/pubmed/16720896 ]