The estrogen receptor (ER) is a ligand-activated transcription factor that mediates the effects of the steroid hormone 17 beta-estradiol, in both males and females. Since the isolation and cloning of ER, the consensus has been that only one such receptor exists. The finding of a second subtype of ER (ER beta) has caused considerable excitement amongst endocrinologists. In this article, we present data regarding the genomic structure and chromosomal localization of the human ER beta gene, demonstrating that two independent ER genes do exist in the human. Furthermore, we present data regarding the tissue distribution of human ER beta, showing that this receptor is expressed in multiple tissues. For instance, ER beta is found in developing spermatids of the testis, a finding of potential relevance for the ongoing debate on the effects of environmental estrogens on sperm counts. In addition, we find ER beta in ovarian granulosa cells, indicating that estrogens also participate in the regulation of follicular growth in the human.