Regulates G protein-coupled receptor signaling cascades. Inhibits signal transduction by increasing the GTPase activity of G protein alpha subunits, thereby driving them into their inactive GDP- bound form. . [Isoform 5]: Behaves as a cell cycle-dependent transcriptional repressor, promoting inhibition of S-phase DNA synthesis. . This is the function of Ensembl gene identifier ENSG00000159788 (RGS12, regulator of G protein signaling 12).