Cell membranes are characterized by a complex lipid composition and asymmetric distribution of lipids (lipids facing the outside of the cell are different from the lipids that are facing the inside of the cell). We developed a model system that captures these features. It is based on supported lipid bilayers (SLBs) on TiO2. They contain the canonical high-melting, low-melting, cholesterol lipid mixture commonly used to model cell membranes, plus phosphatidyl serine (PS), which strongly interacts with TiO2 and is therefore distributed asymmetrically. The strong PS-TiO2 interaction mimics the cytoskeleton-cell membrane interactions that occur in the cell. The goal of this proposal is to quantify lipid composition and distribution in these asymmetric cell membrane mimics by neutron reflectometry.