rev-erbα is transcribed from the opposite strand of the thyroid receptor α (c-erbaα) gene on chromosome 17, with a 269-nucleotide overlap between the two transcripts. the other mammalian isoform of the receptor, rev-erbβ is encoded by another gene on chromosome 3. in addition, there is one non-mammalian homolog, the ecdysone-regulated gene e75, which is present in "drosophila" and "c. elegans". the rev-erbα gene itself has multiple transcripts. two promoters govern the expression of the rev-erbα gene in human and rat, generating two mrna isoforms. the full-length isoform encodes a 614-amino acid protein, while a second isoform is generated from an internal promoter and produces a protein that is shorter by 106 amino acids. both rev-erbα mrna isoforms contain e-boxes as well as rev-erbα response elements, which means that they can be regulated in a circadian manner by the bmal and rev-erba proteins. in fact, both transcripts exhibit rhythmic expression in serum-synchronized fibroblasts.