It is not clear why there should be differential expression of the mitochondrial transcriptome between tissues with higher abundance of transcripts in gill and digestive gland than mantle and foot but this may reflect different Platycodin-D physiological roles of the tissues and different energy demands. Thus in gill, energy demand may be high because of beating cilia that are important in filter feeding and because of its role in osmoregulation. Digestive gland has high energy requirements through its role in intra and extra-cellular digestion and in detoxification. Differences in levels of mitochondrial transcription between tissues has also been observed in mammals. Within each tissue there is also differential abundance of mitochondrial transcripts which is curious since these are produced initially in polycistronic form. The abundance of each transcript appears to be unrelated to its position on the cistron as illustrated by the finding that in each tissue ND4 was the most abundant mitochondrial mRNA even though it is eleventh in order on the polycistron, while ND6 is present at much lower abundance but is located at position 3. Differential transcript abundance might be a Nardosinone requirement of the stoichiometry of the electron chain complexes but Complex 1 in mammals contains equimolar quantities of the seven components encoded by the mitochondrial genome. Transcript abundance will be determined not only by rates of transcription but also by differences in nucleolytic processing, by rates of polyadenylation of pre-mRNA and by rates of degradation. These processes will be regulated by intracellular and extracellular factors and are likely to be affected by environmental stressors. Another observation of considerable interest is the identification of mitochondrial transcripts produced from regions not associated with identified genes. These are found in the Control Region and upstream of the Cox1 ORF. These may be involved in the initiation of DNA replication or control of mitochondrial gene transcription. The tissue distribution of the sex-specific transcripts VERL and VCL are somewhat surprising.