The genome of Mucor racemosus was analyzed to determine the relative levels of codon usage. The codon bias differred from that of Escherichia coli. The active, soluble isoform of NADH cytochrome b5 reductase containing 228 amino acids was successfully overexpressed and secreted using alpha factor in Pichia pastoris under the control of the alcohol oxidase promoter and finally purified. The culture medium and incubation time were optimized, and the maximum expression level observed was about 23 U/ml using X-33 recombinant yeast grown for 120 h with 0.5% (v/v) methanol in complex media.