SLC19A1 encodes the reduced folate carrier, a membrane antiporter that facilitates cellular uptake of folate nutrients and immunoreactive cyclic dinucleotides 1. The protein operates as a secondary active transporter, exporting intracellular organic anions down their concentration gradients to drive substrate import 2. SLC19A1 demonstrates high affinity for N5-methyltetrahydrofolate, the predominant circulating folate form, and mediates transport of antifolate drugs including methotrexate 1. Importantly, SLC19A1 serves as the major transporter for cyclic dinucleotides such as 2'3'-cGAMP, which are critical immune signaling molecules in the cGAS-STING pathway 2. Structural studies reveal distinct recognition mechanisms: cyclic dinucleotides bind as dual-molecule units while folates occupy a separate pocket within the transporter cavity 1. The transporter is ubiquitously expressed and represents the primary folate uptake mechanism in mammalian cells 3. Genetic variants in SLC19A1 are associated with neural tube defects and influence methotrexate toxicity in rheumatoid arthritis patients 45. SLC19A1 deficiency results in folate-responsive megaloblastic anemia and immunodeficiency disorders, highlighting its critical role in both folate metabolism and innate immunity.