Merocyanine 540 is a fluorescent lipophilic probe which binds preferentially to membranes with loosely packed lipids and as such stains apoptotic cells more brightly than normal cells. This change is due to an increase in fluorescence which occurs with the loss of membrane symmetry. Lymphocytes undergoing apoptosis loosen the packing of lipids on their surface. Changes in merocyanine 540 fluorescence have been shown to generally correlate with the translation of membrane phosphatidylserine from the inner to the outer leaflet of the plasma membrane.