**3.2 PRL proteoform pattern in human pituitaries**

The PRL proteoform pattern was found in human pituitaries. Qian et al. [9] found six PRL proteoforms with 2DGE in human pituitaries and then verified four of six PRL proteoforms with 2DGE-based Western blot in human pituitaries (**Figures 1** and **2**). The p*I* and *Mr* of these PRL proteoforms are slightly different. Each PRL proteoform was digested with trypsin, and followed by MS and MS/MS analysis (**Figures 3** and **4**). The characteristic tryptic peptide are calculated to determine whether the signal peptide (position 1–28) in each PRL proteoform (**Table 2**), which was compared to the observed ions of each PRL proteoform. It found all PRL proteoforms all contained the tryptic peptide sequence MNIKGSPWK (position 1–9), which clearly demonstrated that six PRL proteoforms are all PRL prohormone, but not mature PRL.
