Surrogate peptides unique to the target proteins of interest (ER, TET2, GATA3 and Actin) were chosen and stable-isotope-labelled versions (SIS) of these peptides were synthesized.

A background matrix consisting on a MCF7 cell lysate was digested with trypsin. The resulting peptide mixture is then spiked with increasing concentrations of the heavy-labelled peptide standards and a constant concentration of the light version of the same surrogate peptides.

Curve blanks contained no heavy-labelled peptides.