Connexin43 is the major protein of gap junctions in heart and smooth muscle including the myometrium. Molecular cloning of the connexin43 gene reveals a similar organization to that exhibited by other connexin genes: the 5' untranslated region is interrupted by an 8.5 kilobase intron. The promoter region preceding the first exon contains a TATA box and AP-1 and AP-2 sites. In addition, a series of half-palindromic oestrogen response elements is present in this region. When this promoter is linked to the reporter gene luciferase, it drives the expression of luciferase constitutively in HeLa cells transfected with this luciferase-connexin43 promoter fusion construct. When the same cells are cotransfected with oestrogen receptor cDNA, an upregulation of luciferase expression by oestrogen occurs in a cell-specific manner.