Phosphorylation of specific serine or threonine residues is an important post-translational modification for regulating the activity of most proteins. Stimulation of a variety of cell signaling pathways activates the receptor and non-receptor ser/thr kinases that mediate these protein modifications. Antibodies that can detect phosphoserine or phosphothreonine residues are excellent tools for characterizing changes in the post-translational state of a broad range of phosphorylated proteins. Immunoprecipitation of proteins of interest followed by detection of phosphoserine or phosphothreonine using anti-phosphoserine antibody is commonly used to correlate changes in phosphorylation state with alterations in protein activity.