Patent ID: 8772043

Claim:
A method of determining the concentration of a polypeptide in a cell or part thereof, wherein said polypeptide comprises at least one universal peptide tag that is incorporated into the amino acid sequence of said polypeptide, and wherein said polypeptide does not comprise an epitope tag, said method comprising: a. analyzing a sample for the presence of said at least one universal peptide tag in said host, or cell or part thereof, wherein said analysis is performed by mass spectrometry, and further wherein said at least one universal peptide tag remains intact; b. detecting the presence of said polypeptide based on the presence of said at least one universal peptide tag in said sample; c. determining the concentration of said at least one universal peptide tag in said cell, or part thereof; and, d. calculating the concentration of said polypeptide based on the concentration of said at least one universal peptide tag in said sample.