We recently cloned and characterized a novel immediate-early gene, called er1, from Xenopus embryos whose expression levels were increased during mesoderm induction by fibroblast growth factor (FGF). We describe here the isolation and expression pattern of the human er1 sequence. Human ER1 and Xenopus ER1 proteins display 91% similarity; the amino acid sequence motifs, including the putative DNA-binding SANT domain, the predicted nuclear localization signals (NLS) and the putative SH3 binding domain share 100% identity. er1 mRNA expression was negligible in all 50 normal human tissues analyzed. Examination of nine breast carcinoma-derived cell lines and eight breast tumour tissue samples by reverse transcription-polymerase chain reaction (RT-PCR) revealed that human er1 was consistently expressed in all tumour cell lines and tumour tissue while remaining undetectable in normal breast cell lines and breast tissue. These data suggest that er1 expression is associated with the neoplastic state in human breast carcinoma.