Protamine post-translational modification (PTM) site validation by spectral comparison with synthetic peptides. Mass spectra of identified endogenous protamine peptides with novel PTM sites and their synthetic counterparts. Major peaks are labeled in the spectra and the fragment ions indicated in the peptide sequence. A) A novel site of serine acetylation on residue S42 of PRM1. B) A novel site of lysine methylation on residue K49 of PRM2. C) A novel site of serine phosphorylation on residue S55 of PRM2. D) A novel site of serine acetylation on residue S55 of PRM2.