Big Endothelin-1 (BigET) is a peptide of 38 amino acids and is the precursor of Endothelin-1 (ET), represented by amino acids 1-21 (http://www.uniprot.org/uniprot/P05305). ET is a potent vasoconstrictor and is produced by vascular endothelial cells. Accordingly it has a wide tissue distribution (http://www.ncbi.nlm.nih.gov/UniGene/ESTProfileViewer.cgi?uglist=Hs.511899). Serum and plasma are suitable for use in this assay. Note that BigET levels can differ between serum and plasma therefore don't change sample type during studies.