The invention provides, for example, a gene comprising a polynucleotide coding for the polypeptide having the amino acid sequence shown in SEQ ID NO:3. The gene shows amplification in the 9p23-24 region, has a PX domain and PHD finger motifs, plays an important role in cell growth and differentiation, and in tumorigenesis, and is useful in elucidating the pathology of various diseases caused by a protein involved in cell differentiation in malignant tumor or the like, and in the diagnosis and treatment of such diseases.