A chicken oviduct cDNA clone containing the complete open reading frame of the oestrogen receptor (ER) has been isolated and sequenced. The mol. wt of the predicted 589‐amino acid protein is approximately 66 kd which is very close to that of the human ER. Comparison of the human and chicken amino acid sequences shows that 80% of their amino acids are identical. There are three highly conserved regions; the second and third of which probably represent the DNA‐ and hormone‐binding domains of the receptor. The putative DNA‐binding domain is characterised by its high cysteine and basic amino acid content, and the hormone‐binding domain by its overall hydrophobicity. These two domains of homology are also present in the human glucocorticoid receptor (GR) and the product of the avian erythroblastosis virus (AEV) gene, v‐erbA, indicating that c‐erbA, the cellular counterpart of v‐erbA, belongs to a multigene family of transcriptional regulatory proteins which bind steroid‐related ligands. The first highly conserved ER region is not present in the truncated v‐erbA gene, but shares some homology with the N‐terminal end of the GR. The function of the v‐erbA gene product is discussed in relation to its homology with the ER and GR sequences.

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