An estrogen receptor gene and utilization of the estrogen gene are described. The estrogen receptor gene has a nucleotide sequence coding for any of the following amino acid sequences: (a) the amino acid sequence of SEQ ID NO: 1, (b) the amino acid sequence of SEQ ID NO: 4, (c) an amino acid sequence exhibiting 95% or more amino acid identity to the amino acid sequence of SEQ ID NO: 1, and (d) an amino acid sequence exhibiting 95% or more amino acid identity to the amino acid sequence of SEQ ID NO: 4.