Msr-blue emits blue fluorescence after activation by methionine sulfoxide reductase A (Msr A). Msr-blue responds to Msr A in both a time- and dose-dependent manner, with more than a 100-fold increase in emission observed. It is converted to its corresponding sulfide (15′) under catalysis by either purified Msr A or a cell lysate [1]. In a 6-OHDA-treated PC12 cellular model of Parkinson's disease (PD), Msr-blue was employed to probe Msrs function. Using Msr-blue, a decline in Msr activity in the PD model was disclosed for the first time [1].