Required for sterol biosynthesis. Functions as a positive regulator of cytochrome P450 enzymes erg5 and erg11. Function requires bound heme. Interacts with erg5 and erg11. Up-regulated by sre1 in response to absence of oxygen. The cytochrome b5 heme-binding domain lacks the conserved iron-binding His residues at positions 77 and 101. Cells are viable under normal growth conditions, but contains a reduced amount of ergosterol and elevated amounts of the ergosterol biosynthetic intermediates, 24-methylene lanosterol, ergosta-5,7,24(28)-trienol and ergosta-5,7-dienol. Deletion mutant is sensitive to the inhibitors of sterol synthesis itraconazole and CoCl(2). Belongs to the cytochrome b5 family. MAPR subfamily.