The present invention provides a human cytochrome b5 (HCB5) and polynucleotides which encode HCB5. The invention also provides genetically engineered expression vectors and host cells and a method for producing HCB5. The invention also provides for agonists, antisense molecules, antibodies, or antagonists of HCB5, and their use in the prevention and treatment of diseases associated with expression of HCB5. The invention also provides a method for detecting polynucleotides which encode HCB5.