A unique feature of sperm cells is the subdivision of the plasma membrane into well-defined regional domains with different compositions and functions [25] . In addition to the domains described in the Introduction, the cytosolic droplet that is present in a fraction of cauda epididymal mouse sperm constitutes another level of complexity. To analyze the specific distribution of the sperm DRM proteins in the different membrane regions, the already-validated antibodies were used to localize the respective proteins by immunofluorescence. As shown in Figure 3 , DRM proteins were found in the anterior head (IZUMO, CAV2, and FLOT2), midpiece (SLCA3, FLOT2, BSG, and SLC2A3), and principal piece (SLC2A3). TEX101, a protein that localized exclusively to the light buoyant sucrose gradient fractions, was present only in the cytoplasmic droplet.