UAS regulatory sequences drive expression of a reporter comprised of a fusion of pHluorinSE, the Hsap\CD4 transmembrane domain and tdTomato. When localized to the cell surface, the reporter shows both red and green fluorescence. If the reporter is internalized into an acidic compartment such as the endosome, the fluorescence of the pH-sensitive pHluorinSE fluorophore is largely quenched, leading to loss of the green fluorescent signal.