THSD1 (thrombospondin type 1 domain-containing protein 1; also TMTSP) is a 90-100kD type I transmembrane protein that is reminiscent of Unc5h proteins. It is expressed on embryonic endothelial and hematopoietic stem cells, and may be involved in cell-cell interactions. Mature human THSD1 contains a 388aa extracellular domain (ECD) (aa25-412) and a 418aa cytoplasmic region (aa434-851). The ECD possesses three Ig-like domains (aa156-316) and one TSP type-1 domain (aa340-393). There is one potential alternate start site at Met380, and two splice variants, one of which shows a deletion of aa341-393, and a second that shows a 36aa substitution for aa394-851, generating a soluble form. Over aa25-361, human THSD1 is 76% aa identical to mouse THSD1.