This peptide is generated from amino acid residues 4 to 18 of the atrial natriuretic peptide (ANP). It has been used as a model cysteine consisting of peptide in quantitative mass spectroscopic analysis. This fragment is also involved in C-ANP (4-23),(Des-Gln18,des-Ser19,des-Gly20,22,des-Leu21), which is completely selective in discriminating rat C-AMP receptors.