Luminescent proteins originally isolated from marine or terrestrial organisms have played a key role in the development of several biosensing systems. These proteins have been used in a variety of applications including, immunoassays, binding assays, cell-based sensing, high throughput screening, optical imaging, etc. Among the luminescent proteins isolated, the bioluminescent protein aequorin has been one of the proteins at the forefront in terms of its use in a vast number of biosensing systems. In our laboratory, we have employed aequorin as a label in the development of highly sensitive assays through chemical and genetic modifications from single step analysis of physiologically important molecules in biological fluids. An important aspect of optimizing these assays for clinical use involves understanding the stability of the various aequorin variants that are available. To this end we have designed several stability studies involving three important aequorin mutants, Mutant S, Mutant 5, and Mutant 53. The cysteine free aequorin, Mutant S, has been the most ubiquitously used aequorin variant in our laboratory because of its increased stability and activity as compared to native aequorin. Mutant 5 and Mutant 53 contain a single cyteine residue at position 5 and 53 in the protein, respectively. Because of the presence of a single cysteine residue, Mutant 5 and Mutant 53 both can be site-specifically conjugated. This site specific conjugation capability gives Mutant 5 and Mutant 53 an advantage over native aequorin when developing assays. Additional studies optimizing the expression, purification, and charging of aequorin Mutant S were also performed. A thorough understanding of the efficient expression, purification, and storage of these aequorin mutants will allow for the more practical utilization of these mutants in the development of future biosensing systems.