The field of proteomics is presently dominated by the "bottom-up" strategy,where proteins are enzymatically digested into peptides for mass spectrometric identification.While this approach is highly effective at identifying large numbers of proteins present in complex samples,the digestion into peptides renders it impossible to identify the proteoforms from which they were derived.We have developed a powerful new strategy for the identification of proteoforms and the elucidation of proteoform families(groups of related proteoforms)from experimental determination of the accurate proteoform mass and number of lysine residues contained.Accurate proteoform masses are determined by standard LC-MS analysis of undigested protein mixtures in an Orbitrap mass spectrometer,and the lysine count is determined using the recently developed NeuCode isotopic tagging method.We demonstrate the approach in analysis of the yeast proteome,revealing 8,637 unique proteoforms and 1,178 proteoform families.The elucidation of proteoforms and proteoform families afforded here provides an unprecedented new perspective upon proteome complexity and dynamics.