 The originally cloned TP from placenta (343 amino acids in length) is known as the α isoform and the splice variant cloned from endothelium (with 407 amino acids), termed the β isoform.The first 328 amino acids are the same for both isoforms, but the β isoform exhibits an extended C-terminal cytoplasmic domain.Both isoforms stimulate cells in part by activating the Gq family of G proteins.