The estrogen receptor (ER) is the steroid-hormone nuclear receptor that binds estrogen and functions to activate genes involved in a variety of biological processes. Two forms of the ER, ER alpha and ER beta, are encoded by independent genes, and each plays different roles in gene regulation. Additional variants of ER beta have also been reported; these include ERb2, ERb3, ERb4, and ERb5. All forms of the ER have the ability to heterodimerize with each other. Differences in localization and concentrations as well as heterodimerization among the variants may serve to achieve distinct cell and tissue responses. ER alpha generally functions as an activator of transcription while ER beta functions as a repressor.