CYB5R3 is an amphipathic microsomal isoform (one hydrophobic membrane-anchoring domain) and is found in human erythrocytes.
There is an amphipathic microsomal isoform that is found in all cell types but red blood cells; this isoform has one hydrophobic membrane-anchoring domain and one catalytic domain that is hydrophilic. The other isoform, a soluble cytochrome b5 reductase isoform, is found in human erythrocytes. 
