Mediates the Na(+)-independent transport of organic anions such as estrone-3-sulfate (PubMed:10873595). Mediates transport of prostaglandins (PG) E1 and E2, thyroxine (T4), deltorphin II, BQ-123 and vasopressin, but not DPDPE (a derivative of enkephalin lacking an N-terminal tyrosine residue), estrone-3-sulfate, taurocholate, digoxin nor DHEAS (PubMed:16971491). Ubiquitous. Highly expressed in spleen and leukocytes. Generally the expression of isoform 1 is higher than that of isoform 2. Isoform 2 is particularly abundant in testis and brain. In testis, isoform 1 is detected in spermatogonia at different stages and absent from Sertoli cells, while isoform 2 is present in both (at protein level). Expressed in the choroid plexus epithelium, isoform 1 being localized at the basolateral membrane and isoform 2 at the apical one, as well as in the subapical intracellular vesicular compartments. Differential expression of both isoforms is also observed in other brain region: isoform 1 is very abundant in the gray matter of the frontal cortex, but is not associated with neuronal cell bodies. Not detected in the white matter. In contrast, isoform 2 is associated with neuronal bodies and axons in both the gray and the white matters of the frontal cortex. Belongs to the organo anion transporter (TC 2.A.60) family.