Cytochrome b5 is a membrane-bound heme protein that possesses two heme groups and functions as an electron carrier for many membrane-bound oxygenases. Two isoforms of cytochrome b5, a microsomal membrane-bound form and a cytoplasmic form, are produced by alternative splicing. Mutations in the gene encoding cytochrome b5 are associated with Leber’s hereditary optic neuropathy and myopathy. CYB5D1 (cytochrome b5 domain containing 1) is a 228 amino acid protein that exists as two alternatively spliced variants and contains one cytochrome b5 heme-binding domain, suggesting a similar function to cytochrome b5. The gene encoding CYB5D1 maps to human chromosome 17, which comprises over 2.5% of the human genome and encodes over 1,200 genes.