again at exon 9, with the choice of exon 9a, 9b, 9c, or 9d. Evolution of isoform generation In terms of structure, the genes are very similar, suggesting that they arose through gene duplication of an ancestral gene. The most highly related genes are the α- and γ-genes, utilizing two promoters and differing only with the presence of the unique 2a exon in the α-gene. Although substantial differences between alternative exons from the same gene have been revealed by sequence comparison (1a and 1b, 6a and 6b, and the exon 9s), most exons are, however, highly conserved between the different genes. For example, exon 1a and 1b from the α-gene vary considerably in sequence; however, the sequence from exon 1a from the α-, β-, γ-, and δ-genes is highly conserved. Due to the conservative nature of the genes, it is believed that the genes evolved from a common ancestral gene, giving rise to over 40 functionally distinct isoforms. The expression of these isoforms is highly regulated and variable throughout development. The diversity of tropomyosin expression, both in space and in time, provides the potential not only to regulate actin filament function but to create specialised actin filament populations. Spatial sorting of tropomyosin isoforms Numerous reports detail that tropomyosin isoforms are sorted to different intracellular locations, often associating with actin filament populations that are involved in specific processes. Direct visualization of spatial segregation of isoforms was initially observed by Burgoyne and Norman and soon after by Lin and co-workers. They observed that specific isoforms were associated with distinct cellular structures. Using specific antibodies, they were able to identify the presence of both HMW and the LMW isoforms of the γ-gene in stress fibers; however, only LMW isoforms were detected in ruffling membranes. These studies have been extended to a number of cell types with similar results. Extensive studies in neuronal cells, fibroblasts, skeletal muscle and osteoclast cells has further highlighted the complex association tropomyosin isoforms have with cellular structures. These studies have led to the realization that the regulation of isoform sorting is extremely complex and highly regulated. Regulation of sorting Sorting of tropomyosin isoforms in discrete intracellular locations is developmentally regulated. Initial studies reported that the sorting of isoforms changed through development, where Tropomyosin 4 was initially localized to the growth cone of growing neurons, but in mature neurons it was relocated to the somatodendritic compartment. These observations have been