The members of solute carrier family 21 (SLC21)/SLCO are organic anion transporters that facilitate Na+-independent transport of organic anions and cations, including bile acids, steroid conjugates, and thyroid hormones. SLCO4A1/SLC21A12, also known as OATP4A1, is widely detected in the human body at the transcriptional level; it mediates Na+-independent transport of organic anions such as the thyroid hormones and taurocholate.