The present invention provides a partial peptide comprising the functional domain of a protein comprising an amino acid sequence which is the same or substantially the same as an amino acid sequence represented by SEQ ID NO:2 or SEQ ID NO: 4; a screening method and a kit therefor for a substance which regulates ASK1 activation, or a prophylactic or therapeutic substance for diseases associated with apoptosis or inflammation, which comprises using the protein or the partial peptide or a cell which produces the same, and optionally ASK1 or a partial peptide thereof or a cell which produces the same; and an agent for regulating apoptosis or inflammatory cytokine production, or a prophylactic or therapeutic agent for diseases associated with apoptosis or inflammation, which comprises a substance for regulating an activity of a protein comprising an amino acid sequence which is the same or substantially the same as an amino acid sequence represented by SEQ ID NO:2 or SEQ ID NO: 4.