Oftentimes, ELPs are not used in isolation, but are rather fused with other proteins to become functionally active. The structure of these other proteins will have a certain effect on transition temperature.  It is important to be able to predict the transition temperature that these fusion proteins will have relative to the free ELPs, as this temperature will determine the fused protein's applicability and phase transition.  A theoretical model is available that relates the change in T of the fused protein to the varying ratios of each individual amino acid found in the fused protein.  The model involves calculating a surface index (SI) associated with each amino acid and then extrapolating, based on the ratio of each amino acid present in the fused protein, the total change in the T associated with the fusion protein, ΔT:
SI=(ASA/ ASA)(T) 
where ASA refers to the area of the entire fused protein that is available to the solvent that is being used, ASA refers to the area of the guest residue on the ELP that is available to the solvent, and T is the transition temperature that is unique to the amino acid.  Summing up the contribution of each potential guest residue (XAA) will yield an SI index that is directly proportional to ΔT It was found that the amino acids that are charged under a physiological pH of 7.4 have the greatest impact on the overall SI of a fused protein.  This is due to the fact that they are more accessible to water-containing solvents, thereby increasing the ASA and also have high T values.  Hence, knowledge of the transition temperature of a fused protein is highly dependent on the presence of these charged residues.