UAS regulatory sequences drive expression of a membrane-associated pH sensor ('MApHS') designed to analyze phagosome maturation. This consists of the pH-sensitive green fluorescent protein pHluorinE (extracellular), a transmembrane domain (Tag:M(hCD4)) and the tdTomato red fluorescent protein (cytoplasmic). Since pHluorinE loses fluorescence as the pH is lowered (losing fluorescence below pH 6.0), the sensor is expected to lose green fluorescence but retain red fluorescence upon acidification of phagosomes.