Updated project metadata. Unanticipated regulatory protein modifications can be discovered from the unbiased comprehensive analysis of biological systems using SAMPEI. To explore this idea, we sought to map protein modifications induced during mammalian cell differentiation, modeled by the response of mouse RAW264.7 macrophage cells to lipopolysaccharide (LPS), a potent inducer of macrophage activation and differentiation that involves extensive protein and metabolic signaling.