The ubiquitin (Ub) pathway involves three sequential enzymatic steps that facilitate the conjugation of Ub and Ub-like molecules to specific protein substrates. The first step requires the ATP-dependent activation of the Ub C-terminus and the assembly of multi-Ub chains by the Ub-activating enzyme known as the E1 component. The Ub chain is then conjugated to the Ub-conjugating enzyme (E2) to generate an intermediate Ub-E2 complex. The Ub-ligase (E3) then catalyzes the transfer of Ub from E2 to the appropriate protein substrate. A wide range of enzymes facilitate in the proteolytic Ub pathway, including monoclonal nonspecific suppressor factor-beta (MNSF-beta), a subunit of MNSF, which is a lymphokine product of a murine T cell hybrid-oma that restricts the production of LPS-induced immunoglobulin secreting cells in an antigen-nonspecific manner. MNSF-beta is a ubiquitin-like fusion protein consisting of the ribosomal protein S30 and a protein that shares 36% sequence identity with ubiquitin. This ubiquitin-like segment (Ubi-L) can be cleaved from MNSF-beta in the cytosol.