Likewise, a novel isoform of ERβ, termed ERβ2, containing an in-frame insertion of an exon of 54 nucleotides, resulting in an insertion of 18 amino acids in the LBD, was recently identified first by screening rat prostate cDNA library, and is also expressed in human cell lines. 43 ERβ2 binds E 2 with lower affinity (Kd = 8 nM) than ERβ1 (Kd = 1 nM). At least 10 splice variants of ERβ have been identified. 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55
Sex hormone-binding globulin (SHBG) is thought to mainly function as a transporter and reservoir for the estradiol and testosterone sex hormones. However it has also been demonstrated that SHBG can bind to a cell surface receptor (SHBG-R). The SHBG-R has not been completely characterized. A subset of steroids are able to bind to the SHBG/SHBG-R complex resulting in an activation of adenylyl cyclase and synthesis of the cAMP second messenger.  Hence the SHBG/SHBG-R complex appears to act as a transmembrane steroid receptor that is capable of transmitting signals to the interior of cells.