By screening a lung cDNA library using rat T1a as probe, Ma et al. (1998) cloned human T1A, which they designated T1A2. The deduced 162-amino acid T1A2 protein contains a putative transmembrane region, a protein kinase A (see 176911) phosphorylation site, and a protein kinase C (see 176960) phosphorylation site. Northern blot analysis detected a 1.2-kb T1A2 transcript only in lung. Ma et al. (1998) also identified a potential T1A variant, which they called T1A1. Zimmer et al. (1999) identified several cDNAs encoding the human homolog of mouse gp38, rat gp40, and dog gp40, and by 5-prime RACE and RT-PCR of placenta mRNA, they cloned full-length GP36. The deduced 162-amino acid protein is a typical type I membrane glycoprotein, with an N-terminal signal sequence, a signal sequence cleavage site, a C-terminal hydrophobic stretch, and a short cytoplasmic tail. It also contains 31 potential O-glycosylation sites. Most sequence conservation between the human, rat, mouse, and dog proteins resides in the C-terminal tail, and 15 O-glycosylation sites are conserved in all 4 species. GP36 has a calculated molecular mass of 17.8 kD, and the predicted mature protein has a calculated molecular mass of 15.6 kD. PCR analysis detected GP36 transcripts in placenta, lung, skeletal muscle, and heart. Expression was weaker in brain, kidney, and liver, and was not detected in pancreas. Western blot analysis of transfected canine kidney cells recognized a major protein of 36 kD and minor proteins of about 28 and 70 kD. GP36 was expressed on the cell surface. Immunohistochemical analysis of GP36 in placenta, kidney, lung, and nasal polyps showed expression at the apical plasma membrane of vascular endothelial cells and in alveolar epithelial cells. Schacht et al. (2003) found that mouse T1a was expressed in the cardinal vein and in budding Prox1 (601546)-positive lymphatic progenitor cells between embryonic days 10.5 and 11.5. Expression became restricted to lymphatic endothelium during later development. Ultrastructural analysis revealed prominent localization to the luminal plasma membrane of lymphatic vessels.