Proteolysis of cellular proteins is carried out by a complex cascade of enzymes and displays a high degree of specificity towards its numerous substrates. Discovery and studies of the ubiquitin-proteasome pathway reveal that the specific protein degradation is a highly complex, temporally controlled and tightly regulated process which plays important roles in a broad array of basic cellular processes. A defective protein, which binds poly-ubiquitin chain (so-called “degrading label”) through its ε-amino group of Lys residue, is degraded in the proteasome. This selectively cellular elimination of defective protein is a key process in protein kinesis.