UQCRFS1  Ubiquinol-cytochrome c reductase, Rieske iron-sulfur polypeptide 1, also known as UQCRFS1, Rieske iron-sulfur (Fe-S) protein, Cytochrome b-c1 complex subunit 5, or Complex III subunit 5 is a protein which in humans is encoded by the "UQCRFS1" gene. UQCRFS1 is a subunit of the respiratory chain protein Ubiquinol Cytochrome c Reductase (UQCR, Complex III or Cytochrome bc1 complex), which consists of the products of one mitochondrially encoded gene, MTCYTB (mitochondrial cytochrome b) and ten nuclear genes UQCRC1, UQCRC2, Cytochrome C1, UQCRFS1 (this protein, a type of Rieske protein), UQCRB, ("11kDa protein"), UQCRH (cyt c1 Hinge protein), UCRC ("cyt. c1 associated protein"), and ("Rieske-associated protein"). "UQCRFS1" is located on the q arm of chromosome 19 in position 12, has 2 exons, and spans 5,969 base pairs. The "UQCRFS1" gene produces a 29.7 kDa protein composed of 274 amino acids. UQCRFS1 is a subunit of the respiratory chain protein Ubiquinol Cytochrome c Reductase (UQCR, Complex III or Cytochrome bc1 complex). The structure of the complex is a symmetric homodimer composed of one mitochondrial genome encoded cytochrome b subunit and ten other nucleus encoded subunits. The primary structure of UQCRFS1 from cDNA analysis is composed of a 78 amino acid long N-terminal extension sequence. The "UQCRFS1" gene encodes for an iron-sulfur protein, which is an essential subunit of the Ubiquinol Cytochrome c Reductase or Complex III in the mitochondrial respiratory chain. Complex III is responsible for electron transfer from coenzyme Q to cytochrome c as well as the proton transfer from the extracellular matrix to the intermembrane space which leads to ATP-coupled electrochemical potential generation. Incorporation of the subunit UQCRFS1 is the second to last step in complex III assembly.