We review the evidence that the function of the SLC25A19 gene product, previously identified as the mitochondrial deoxyribonucleotide carrier (DNC), is actually the transport of thiamine pyrophosphate. This evidence comes from enzyme kinetics, homologous yeast protein alignments, gene knockout studies, and clinical samples from Amish Microcephaly patients. This diverse body of evidence consistently points to the conclusion that SLC25A19 is not the true mitochondrial DNC gene. The identification of the correct mitochondrial DNC is important for research on the genetic diseases of mitochondrial DNA maintenance and the toxicity experienced by many HIV patients undergoing antiretroviral therapy involving nucleoside analogs.