The PEA3 group is a homogeneous group of the ets transcription factor family and is composed of three known members, PEA3, ERM and ER81, which, on the amino acid (AA) level, are more than 95% identical within the DNA-binding domain (the Ets domain), more than 85% within a 32 AA domain (the acidic domain) localized in the amino-terminus and almost 50% identical overall. By screening a human kidney cDNA library with a specific probe obtained from mouse ER81, we isolated two clones of 1.6 and 1.5 kb in length encoding a 458 AA open reading frame. When compared to mouse ER81, the present human ER81 lacks the last 13 AA of the acidic domain and the 5 AA contiguous to the carboxy-terminal part of the acidic domain. Of the 458 AA of the human ER81 protein, 97% are identical to mouse ER81. Gel shift analysis indicates that the full-length human ER81 protein is able to bind specifically to an oligonucleotide containing the binding sites recognized by most of the Ets proteins. By transient expression in RK13 mammalian cells, human ER81 protein transactivated a reporter plasmid containing Ets binding sites, indicating that this molecule is a bonafide transcriptional activator, while by expression in Cos-1 transfected cells, we detected the presence of human ER81 protein in the nucleus using immunocytochemistry. In human tissues, ER81 mRNA is very highly expressed in brain, highly expressed in testis, lung and heart, moderately in spleen, small intestine, pancreas and colon, weakly in liver, prostate and thymus, very weakly in skeletal muscle, kidney and ovary and not in placenta and peripheral blood leukocytes. Analysis of human solid or haematopoietic tumour cell lines showed that most of them did not express ER81 detectably. Database searches revealed that ETV1 mRNA is highly similar to human ER81 described here, although it contains the full-length acidic domain present in mouse ER81. By screening a genomic DNA library, we characterized the intron-exon junction within the acidic domain of human ER81 and we showed that this junction corresponds to the site where the remaining AA of the acidic domain of ETV1 or mouse ER81 are inserted.