This procedure describes how to assess the native quaternary structure of membrane proteins by PAGE using mild solubilization in the non-dissociative detergent perfluoro-octanoic acid (PFO). The number of subunits is deduced from the molecular mass of the preserved homo-multimeric protein complex. The PFO-PAGE technique was first described by Ramjeesingh, M et al. (1999) and applied later to determine the subunit stoichiometry of other channels (e.g. Kedei, N. et al. (2001) and in the present study to Orai and P2X2 expressed in Drosophila S2 cells or human embryonic kidney HEK293 cells).