arrays of actin filaments. There is no evidence for active transport of tropomyosin isoforms to specific locations. Rather, it appears that sorting is the result of local assembly of preferred isoforms at specific intracellular site. The mechanisms that underlie tropomyosin isoform sorting appear to be inherently flexible and dynamic in nature. Isoforms are not functionally redundant Many studies have led to the understanding that tropomyosins perform essential functions and are required in a diverse range of species from yeast, worms, and flies to complex mammals. The essential role of tropomyosins was discovered in the Bretscher laboratory, where researchers found that, by eliminating the TPM1 gene of budding yeasts, growth rates were reduced, the presence of actin cables disappeared, defects in vesicular transport were observed, and mating of the yeast was poor. When a second yeast gene, TPM2, was deleted, no observable changes in the phenotype were recorded; however, when deleted in combination with TPM1, it resulted in lethality. This suggests that TPM1 and -2 genes have overlapping function; however, TPM2 cannot fully compensate of the loss of TPM1, indicating that some functions of TPM1 are unique. Similar results have been observed in flies, worms, amphibians, and mammals, confirming previous results and suggestive of tropomyosin's being involved in a wide range of cellular functions. However, the three co-expressed TMP1, 2, and 4 genes cannot compensate for deletion of the TPM3 gene in embryonic stem cells and preimplantation mouse embryos. Results from gene knockout experiments can be ambiguous and must be carefully examined. In studies in which the deletion of a gene leads to lethality, it can at first appear that the gene product had a truly unique role. However, lethality can also be the result of the inability of the compromised cell to express other isoforms to rescue the phenotype because the required isoform is not naturally expressed in the cell. Specific roles and functions Influencing binding of actin binding proteins to actin filaments The actin microfilament system is the fundamental cytoskeletal system involved in the development and maintenance of cell morphology. The ability of this system to readily respond to cellular cues and undergo structural re-organisation has led to the belief that this system regulates specific structural changes within different cellular regions. Within humans, there are only six actin isoforms, and these isoforms are responsible for an array of unique and complex cellular structures and key cellular interactions. It is