The invention relates to a method for identifying and quantifying a polypeptide from a
library of polypeptides. The method comprises the steps of: 1 - providing a polypeptide
library and a detection tag
library, 2 - generating a nested library comprising the polypeptides and the detection tags, 3 - sequencing the nested library, 4 - selecting a member of the nested library in one or several selection steps that are independent of a physical
genotype-
phenotype linkage, 5 - isolating the detection tag from the selected polypeptide, 6 - identifying and quantifying the detection tag by
mass spectrometry, 7 - obtaining the sequence of the selected polypeptide.