FLIPR(R) membrane potential dye measures changes of charges across the cell membrane, upon activation of SLC1A1. The assay allows the detection of ion channel and transporter modulation by increasing or decreasing the fluorescent signal as cellular membrane potential changes. When cells are depolarized dye enters the cells, causing an increase in fluorescent signal, conversely, cells hyperpolarization results in dye exit and decreased fluorescence. SLC1A1 is a sodium-dependent, high-affinity amino acid transporter that mediates the uptake of L-glutamate and L-aspartate/D-aspartate. It is also reported to mediate the transport of L-cysteine. It functions as symporter that transports one amino acid molecule together with two or three Na+ ions, thus it is an electrogenic transporter eligible to be studied using the membrane potential dye.