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The important role of estrogens in women in physiological and pathological processes is well accepted, but recently it has become evident that estrogens are also important in male physiology, in particular, within bone metabolism and reproduction. Consequently, it is necessary to identify and to characterize the molecular mechanisms of estrogen action in order to evaluate how the pleiotropic effects of estrogens are mediated in a variety of tissues. We have recently shown that human estrogen receptor α (ERα) mRNA is transcribed from at least six different promoters (1A–1F). Transcription of ERα in bone is exclusively dependent on the F-promoter. To study the regulation of ER expression in this tissue, we examined 1 kbp of the F-promoter region of human ERα, which is located more than 70 kbp upstream of the transcription start site of the ERα gene. Transient transfection experiments demonstrated a basal activity from the F-promoter, which was further increased when ERα was cotransfected. We have shown recently that the F-promoter can give rise to at least two ERα isoforms in bone. On the contrary, ERβ expression in primary osteoblasts is extremely low, indicating that this ER isoform plays only a minor role in these cells. In contrast to bone, we have demonstrated that both ERα and ERβ transcripts are readily detected in testis. Here, we report that besides ERα, ERβ transcripts can give rise to two protein isoforms and that this complex situation could have important functional consequences for the signalling of estrogens and their analogs.