The steroid hormone estrogen profoundly influences growth and differentiation programs in the reproductive tract of cycling and pregnant mamals. It is thought that estrogen exerts its cellular effects by regulating the expression of specific target genes. We utilized a messenger RNA differential display method to identify the genes whose expression is modulated by estrogen in the preimplantation rat uterus. Here we report the cloning of a novel gene (ERG1) that is tightly regulated by estrogen in two key reproductive tissues, the uterus and oviduct. Spatio-temporal analyses reveal that ERG1 mRNA is expressed in a highly stage-specific manner in the uterus and oviduct, and its expression is restricted to the surface epithelium of both of these tissues. Nucleotide sequence analysis of the full-length ERG1 cDNA indicates that it has an open reading frame of 1821 nuceotides encoding a putative protein of 607 amino acids with a single transmembrane domain and a short cytoplasmic tail. The extracellular part of the protein contains several distinct structural motifs. These include a zona pellucida binding domain, which is present in a number of proteins such as the zona pellucida sperm binding proteins, and uromodulin, In addition, there is a repeat of a motif called CUB domain, which exists in a number of genes involved in development and differentiation such as bone morphogenetic protein 1 (BMP1). Although the precise function of ERG1 eludes us presently, its unique pattern of expression in the uterus and oviduct and its regulation by estrogen, a principal reproductive hormone, lead us to speculate that this novel gene plays an important role in events during the reproductive cycle and early pregnancy.The steroid hormone estrogen modulates the structure and function of the female reproductive tissues, such as the uterus and oviduct, by eliciting an array of biochemical responses in these tissues. Estrogen critically influences the transport of the fertilized egg through the oviduct into the uterus (1, 2). Estrogen also promotes the growth, differentiation, and remodeling of the uterus at various physiological states, such as the reproductive cycle and pregnancy (3-6). The cellular actions of this hormone are mediated through its nuclear receptors, which function as ligand-inducible transcription factors (7-10). Previous studies in rodents employing immature and ovariectomized model systems have demonstrated that in the uterus, estrogen modulates the expression of genes that are likely to be involved in the regulation of cell growth and proliferation. These include the genes encoding protooncogenes, such as c-fos and c-myc, growth factors, such as epidermal growth factor and insulin-like growth factor-1, and their receptors (11-21). The identity of the majority of estrogen-regulated genes that mediate the uterotropic hormonal responses to estrogen, however, remains largely unknown. In order to understand the role of estrogen in complex physiological processes, such as the changes in the endometrium during the reprodu...