Recent studies by several research groups have shown that G protein estrogen receptor-1 (GPER) formerly known as GPR30, mediates 17beta-estradiol (E2) activation of signal transduction pathways in a variety of human cancer cells and displays E2 binding typical of a membrane estrogen receptor. However, the importance of GPER as an estrogen receptor has been questioned by Otto and co-workers. Some of the pitfalls in investigating the functions of recombinant steroid membrane receptors that may explain the negative results of these investigators are discussed. The characteristics of GPER have also been investigated in a teleost fish, Atlantic croaker, where it has been shown to mediate E2 inhibition of oocyte maturation. Investigations on newly discovered homologous proteins from distantly related vertebrate groups are valuable for determining their fundamental, evolutionarily conserved functions. Therefore, the functions of croaker and human GPERs were compared. The comparisons show that croaker and human GPER have very similar estrogen binding characteristics, typical of estrogen membrane receptors, and activate the same estrogen signaling pathways via stimulatory G proteins (Gs) resulting in increased cAMP production. These results suggest that the estrogen binding and estrogen signaling functions of GPER arose early in vertebrate evolution, prior to the divergence of the teleosts from the tetrapods, more than 200 million years ago. The finding that estrogen membrane signaling through GPER has been conserved for such a long period in two distantly related vertebrate groups, mammals and fish, suggests that this is a fundamental function of GPER in vertebrates, and likely its major physiological role. Copyright 2009 Elsevier Inc. All rights reserved.