The present invention relates to an estrogen receptor gene, characterized by comprising a nucleotide sequence coding for any one of the following amino acid sequences: (a) the amino acid sequence of SEQ ID NO:1, (b) an amino acid sequence of a protein having an estrogen receptor activity, said amino acid sequence has at least 90% sequence identity with the amino acid sequence of SEQ ID NO:1, (c) an amino acid sequence encoded by the nucleotide sequence of SEQ ID NO:2, and (d) an amino acid sequence of a protein having an estrogen receptor activity, said amino acid sequence is encoded by a nucleotide sequence having at least 90% sequence identity with a DNA having the nucleotide sequence of SEQ ID NO:2, and the like. The estrogen receptor gene and the like can be applied to assay systems for evaluating the ability of chemical substances to regulate the estrogen receptor activity.