The consumption of vitamin B12 (B12), also known as cobalamin, is essential for humans. B12 is produced naturally by select bacteria and organisms must acquire the vitamin through their diet. Transport and delivery of B12 through the gastrointestinal tract is dependent on three primary carrier proteins: haptocorrin (HC), intrinsic factor (IF), and transcobalamin II (TCII), each responsible for carrying a single B12 molecule. All three transport proteins bind to B12 with high affinities, but the specificity varies. IF shows the highest specificity for B12, followed closely by TCII, with HC have a broad substrate base including B12 analogs such as cobinamides. Few peptide/protein-based drugs have the ability to survive the gastrointestinal tract and/or cross the intestinal wall to make it to the systemic circulation. Researchers can "hijack" this pathway to deliver B12-drugs in an oral manner. The photocleavable-B12 system was then used as a platform to selectively deliver drugs.