Steroid hormone receptors are a class of cell-specific trans-acting transcription regulatory factors whose activity is controlled by specific binding of the hormone. The hormone-receptor complex appears to associate with promoter/enhancer elements of specific target genes, resulting in activation of transcription (see refs 1 and 2 for reviews). Sequence comparison between the oestrogen^3,4, glucocorticoid^5,6 and progesterone receptors (refs 7,8 and unpublished results) and site-directed mutation analysis^9–11, has identified in each at least two functional domains important for steroid receptor function. Region E (Fig. la), is the hormone-binding domain⁹; region C is a 66-amino-acid region (Figs la,b) that is more highly conserved than the hormone-binding domain and has the potential to form at least two zinc-stabilized 'DNA-binding fingers' analogous to those proposed for the Xenopus transcription factor TFIIIA^7,12,13. We^3,4 and others¹⁴ have suggested that this region may be the receptor's DNA-binding domain. We show here that point mutations replacing two cysteines by two histidines in the first potential DNA-binding finger of the human oestrogen receptor (hER) prevent it from activating gene transcription. We further show that a chimaeric receptor formed by replacing this 66-amino-acid region of the HER with that of the human glucocorticoid receptor (hGR) activates expression of a glucocorticoid-inducible gene, but not of an oestrogen-inducible gene, in the presence of oestradiol. Thus, region C determines the receptor's specificity for target genes.