To identify a domain in HIF-1.alpha. protein, which participates in
stabilization of a fused protein, DNA encoding the following polypeptide
(A) or (B) is provided:(A) a polypeptide having the amino acid sequence
of SEQ ID NO: 1(B) a polypeptide having an amino acid sequence comprising
at least 16 amino acid residues in the amino acid sequence of SEQ ID NO:
1, and imparting stability dependent on an oxygen concentration to other
protein in a cell harboring a fused protein, when the polypeptide is
fused with a nuclear localization signal and the other protein to form
the fused protein.