A major mechanism that cells use to regulate protein function is by phosphorylation and/or dephosphorylation of serine, threonine, and tyrosine residues. Phosphopeptide mapping of these phosphorylated residues allows investigation into the positive and negative regulatory roles these sites may play in vivo. In addition, phosphopeptide mapping can uncover the specific phosphorylated residue and, hence, kinase recognition sites, thus helping in the identification of the relevant kinase(s) and/or phosphatase(s).