Solute carrier family 21/O (SLC21/SLCO) is a group of organic anion transporters, which facilitate Na+-independent transport of organic anions and cations, including bile acids, steroid conjugates, and thyroid hormones. SLCO6A1/SLC21A19, also known as OATP6A1, has been detected at the transcriptional level in various human tissues such as those of the testis, brain, spleen, and placenta. In addition, OATP6A1 mRNA has been detected in several tumor tissues, including those of the lung, bladder, and esophagus. Substrates of OATP6A1 remain unknown.