(A) Monolayer compression isotherms of pure DPPC, pure GM1, and binary mixtures of 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, and 3:7 mol ratio DPPC/GM1 at 30°C (B) Surface pressure at which condensed (C) domains appear in monolayers composed of DPPC and GM1, plotted as a function of GM1 concentration. Domains were visualized using fluorescence microscopy with the TR-DHPE probe partitioning into the more fluid phase.