ACC is the rate-limiting (committed) step in fatty acid synthesis. There are two major isoforms of ACC in mammalian tissues. These are identified as ACC1 (also called ACCα) and ACC2 (also called ACCβ). The ACC1 gene (symbol: ACACA) is located on chromosome 17q12 and is composed of 63 exons that undergo alternative splicing to yield five splice variant mRNAs that generate precursor proteins from 2268 to 2383 amino acids in length. Transcriptional regulation of ACACA is effected by three promoters (PI, PII, and PIII), which are located upstream of exons 1, 2, and 5A, respectively. The PI promoter is a constitutive promoter, the PII promoter is regulated by various hormones, and the PIII promoter is expressed in a tissue-specific manner. The presence of the alternatively spliced exons does not alter the translation of the ACC1 protein which starts from an AUG present in exon 5. The ACC2 gene (symbol: ACACB) is located on chromosome and is composed of 56 exons that encode a precursor protein of 2,458 amino acids.