Intracellular peptides likely regulate several biological processes. EL28 is a new functional intracellular peptide from the human 19S ATPase regulatory subunit 4. EL28 increases in extracts of HeLa cells that are treated with interferon gamma. EL28 activates the proteasome and potentiates interferon gamma activity. EL28 has biotechnological potential because it stimulates the immune response.