of proteins from a single gene. This plays a crucial role in the functionality of higher eukaryotes, with humans expressing more than 5 times as many different proteins (isoforms) as genes through alternative splicing. From a mechanistic point of view, it is much easier for an organism to expand on a current gene/protein family (creating protein isoforms) than it is to create an entirely new gene. From an evolutionary point of view, tropomyosins in higher eukaryotes are notable in retaining all 4 of the potential genes produced by the dual genomic duplication event that took place in early eukaryotic evolution. Genes and isoforms (isoform complexity) Within mammals, four genes are responsible for generating more than 40 different tropomyosin isoforms. In terms of structure, the genes are very similar, suggesting that they arose through gene duplication of an ancestral gene. In humans, these genes are no longer linked and are widely dispersed. In humans, the α1-, β-, α3-, and α4-genes are formally known as TPM1, TPM2, TPM3, and TPM4 and are located at 15q22, 9p13, 1q22 and 19p13, respectively. An alternative nomenclature names the four genes (α,β,γ,δ). Isoforms are defined as highly related gene products that perform, in essence, similar biological functions, with variations existing between the isoforms in terms of biological activity, regulatory properties, temporal and spatial expression, and/or the intercellular location. Isoforms are produced by two distinct mechanisms, gene duplication and alternative splicing. The former mechanism is a process by which multiple copies of a gene are generated through unequal crossing over, through tandem duplication, or by translocation. Alternative splicing is a mechanism wherein exons are either retained in the mRNA or targeted for removal in different combinations to create a diverse array of mRNAs from a single pre-mRNA. Splicing A vast array of tropomyosin isoforms are generated by using a combination of different genes and alternative splicing. In mammals, regardless of the gene, transcription is initiated at the start of either exon 1a or exon 1b. Depending on the promoter and initial exon used, tropomyosin isoforms can be categorized as either high-molecular-weight (HMW, 284 amino acids) or low-molecular-weight (LMW, 248). HMW isoforms express exon 1a and either 2a or 2b, while LMW isoforms express exon 1b. To date, all known tropomyosins contain exons 3-9. Alternative splicing can occur at exon 6, with the mutually exclusive choice of exon 6a or 6b. At the c-terminus, the transcript is spliced