The human chemoattractant receptor-homologous molecule (CRTH2, GPR44, G protein-coupled receptor 44) maps to chromosome 11q12-q13.3 and encodes a 472 amino acid G protein-coupled leukocyte chemoattractant receptor. Chemoattractant receptors present on Th2 cells respond to parasites and play a central role in allergic inflammation; they are absent on type 1 T helper (Th1) cells, which address intracellular bacteria and many viruses. CRTH2 contains seven putative transmembrane domains and mediates signals to the interior of the cell upon exposure to its cognate ligand prostaglandin (PG) D2, which is able to attract basophils, eosinophils, type 2 Th (Th2) cells and type 2 cytotoxic (Tc2) CD8+ T lymphocytes. CRTH2 expression on active Th2 cells influences supportive roles in Th2-type immune reactions. 3.5 kb CRTH2 transcripts are present in thalamus, frontal cortex, pons, hippocampus, hypothalamus and caudate, while 3.4 kb transcripts are present in fetal liver, leukocytes and thymus.