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Though highly expressed in various tissues such as adipose and breast tissue, THTR‐2 is the major absorptive transporter for thiamine in the intestine [3]. It is located on the apical membrane of intestinal epithelia and renal proximal tubule cells, where it plays a role in renal reabsorption of thiamine. In contrast, while exhibiting low ubiquitous expression across a variety of tissues, THTR‐1 is perhaps most physiologically relevant in the bone marrow, ear, and pancreas/liver, as suggested by the manifestation of megaloblastic anemia, diabetes, and sensorineural deafness in patients with loss‐of‐function THTR‐1. Both THTR‐1 and THTR‐2 are expressed at the blood–brain barrier. In accordance with its role as a neurotransmitter transporter, PMAT is most highly expressed in the brain, and notably in the hypothalamus, cortex, substantia nigra, cerebellum, amygdala, and hippocampus regions, as well as in the heart, kidney, skeletal muscle, and liver [4] (Table 2.1).