Cytochrome b is a protein in the mitochondria (see schematic below #9) of eukaryotic cells. The mitochondria is a double layered organelle found in eukaryotic cells (see TEM image below).
Schematic of an animal cell from Wikipedia (https://en.wikipedia.org/wiki/Mitochondrion)
TEM image of mitochondria of a mammal cell from Wikipedia (https://en.wikipedia.org/wiki/Mitochondrion)
Cytochrome b is a component of respiratory complex III. It is a protein that functions as part of the electron transport chain. The electron transport pumps protons to create a proton motive force. Proton gradient is used for the generation of adenosine triphospate (ATP). ATP is an organic compound that provides energy to eukaryotic cells.
Mitochondrial cytochrome b is among the most extensively sequenced genes to date across the vertebrates.
In fact, I am using previously published cytochrome b data from Longfin Smelt (Spirinchus thaleichthys) we designed primers and probes for qPCR. I used Geneious to align sequences downloaded from the sequence database, NCBI (National Center for Biotechnology Information).
Once I had a consensus sequence based on previously published cytochrome b genomic data, my mentor from the University of Alaska Fairbanks, Andres Lopez designed two qPCR assays for us to test out.