supported by studies on different tropomyosin isoforms, showing how tropomyosin populations were relocated during neuron maturation. This evidence is supportive of the notion that tropomyosin isoforms are subject to temporal regulation. Additional studies have identified the role the cell cycle plays in isoform sorting. A study that screened a range HMW products from the α- and β-genes and compared localisation with LMW products from the γ-gene found that the HMW and LMW products are mutually exclusively segregated during the early G1 phase of the cell cycle. Mechanism of sorting While studies suggest that tropomyosin sorting may be influenced by the sorting of mRNAs, there is no absolute correlation between mRNA and protein location. In neurons, Tropomyosin 5NM1 mRNA was found to sort to the pole of the neuron elaborating an axon prior to morphological differentiation. The sorting of Tropomyosin 5NM1/2 mRNA to this location correlated with the expression of the Tropomyosin 5NM1/2 protein. In contrast, the mRNA encoding the Tropomyosin Br2 protein was excluded from the pole of the neuron. The link between mRNA sorting and protein location has been tested in transgenic mice models. The models were created so that the coding regions of Tropomyosin 5NM1/2 and Tropomyosin 3 were expressed under the control of the β-actin promoter with the a β-actin 3'-untranslated region lacking targeting information. The study found that Tropomyosin 3, an isoform that is not normally expressed in neuronal cells, was broadly distributed throughout the neuron, while exogenous expression of the neuronal isoform Tropomyosin 5NM1/2 was found to sort to the growth cone of neurons as does the endogenous Tropomyosin 5NM1/2. As these two transgenes differ only in the tropomyosin coding region yet are localized in two distinct areas, the findings suggest that, in addition to mRNA sorting, the proteins themselves contain sorting information. Studies suggest that tropomyosin isoform sorting may also be influenced by the actin isoform composition of microfilaments. In myoblasts, overexpression of γ-actin resulted in the down-regulation of β–actin and the removal of Tropomyosin 2 but not Tropomyosin 5 from stress fibers. It was later found that, when cells were exposed to cytochalasin D, a chemical that results in the disorganization of actin filaments, tropomyosin isoform sorting was disrupted. Upon the washing out of cytochalasin D, tropomyosin isoform sorting was re-established. This is suggestive of a strong relationship between the process of tropomyosin isoform sorting and the incorporation of tropomyosin isoforms into organized