This kit is designed for enrichment of cellular proteins conjugated with lysine 48-linked polyubiquitin chains using GST-S5a (UIM). Polyubiquitinated proteins bound on GST-S5a (UIM) can be precipitated using gluta-thione resin and eluted by a buffer containing 10 mM glutathione. Polyubiquitinated proteins can be as-sayed by immunoblotting or mass spectrometry.