Erythrocyte membranes extracted in 0.5% Tx-100 contain twice the amount of protein in DRM fraction 2 compared with membranes extracted in 1% Tx-100, whereas those extracted in 2% Tx-100 do not contain any detectable protein. Protein concentrations in fractions obtained from erythrocyte membranes by Tx-100 extraction and sucrose density gradient centrifugation were measured as described in “Materials and methods.”