GNAS gives rise to multiple imprinted gene products, including the α-subunit of the stimulatory G protein (Gsα) and its variant XLαs. Based on genomic sequence, the translation of XLαs begins from the middle of a long open reading frame, suggesting the existence of an N-terminally extended variant termed extralarge XLαs (XXLαs). Although XXLαs, like Gsα and XLαs, would be affected by most disease-causing GNAS mutations, its authenticity and biological significance remained unknown. Here we identified a mouse cDNA clone that comprises the entire open reading frame encoding XXLαs. Whereas XXLαs mRNA was readily detected in mouse heart by RT-PCR, it appeared virtually absent in insulinoma-derived INS-1 cells. By Northern blots and RT-PCR, XXLαs mRNA was detected primarily in the mouse brain, cerebellum, and spleen. Immunohistochemistry using a specific anti-XXLαs antibody demonstrated XXLαs protein in multiple brain areas, including dorsal hippocampus and cortex. In transfected cells, full-length human XXLαs was localized to the plasma membrane and mediated isoproterenol- and cholera toxin-stimulated cAMP accumulation. XXLαs-R844H, which bears a mutation analogous to that in the constitutively active Gsα mutant Gsα-R201H (gsp oncogene), displayed elevated basal signaling. However, unlike Gsα-R201H, which mostly remains in the cytoplasm, both XXLαs-R844H and a constitutively active XLαs mutant localized to the plasma membrane. Hence, XXLαs is a distinct GNAS product and can mimic Gsα, but the constitutively active XXLαs and Gsα mutants differ from each other regarding subcellular targeting. Our findings suggest that XXLαs deficiency or hyperactivity may contribute to the pathogenesis of diseases caused by GNAS mutations.