The structure of the MitoPT reagent allows it to easily penetrate cells and healthy mitochondria. Once inside a healthy non-apoptotic cell, the lipophilic MitoPTreagent, bearing a delocalized positive charge, enters the negatively charged mitochondria where it aggregates and fluoresces red (7). These aggregates, first described by Jelley in 1937, are referred to as J-aggregates (8). When the mitochondrial DY collapses in apoptotic cells, the MitoPT reagent no longer accumulates inside the mitochondria. Instead, it is distributed throughout the cell. When dispersed in this manner, the MitoPT reagent assumes a monomeric form, which fluoresces green (9). Use of the MitoPT kit allows the easy distinction between non-apoptotic red fluorescent cells and apoptotic green fluorescent cells.