(A) Schematic of the in vitro FRET assay to detect co-distribution of phosphatidylserine (PS) and accessible cholesterol in membranes. Liposomes containing PS and cholesterol were mixed with an accessible cholesterol biosensor (ECFP-D4H) and a PS biosensor (mVenus-Lact-C2). ECFP-D4H was excited at 430 nm and emission from mVenus-Lact-C2 due to FRET between ECFP and mVenus was recorded at 525 nm. 