TPE-MI is non-fluorescent until it binds to a thiol group through its maleimide group. Its fluorescence is activated when it labels free cysteine thiols, which are typically hidden inside globular proteins but become exposed when the proteins unfold^[1].

In live HeLa cells, TPE-MI at 50 μM for up to 60 minutes displays a uniform cytoplasmic labeling pattern, with less labeling in the nucleus and a concentration near the endoplasmic reticulum, a key site for protein synthesis and folding^[1].

In cells expressing the mutant 97Q form of Httex1 at high levels, there is an increased TPE-MI fluorescence signal compared to cells expressing the non-disease causing 25Q form of Httex1^[1].

TPE-MI combines the aggregation-induced emission (AIE) fluorogen tetraphenylethane (TPE) with the thiol-reactive maleimide (MI) group, featuring both AIE properties and selective reactivity towards thiols^[2].