Abstract: The invention relates to a plasma membrane marker for identifying fenestrae. The invention also relates to a method of visualizing fenestrae utilizing a plasma membrane marker and light microscopy. The invention also relates to a method of identifying a plasma membrane marker for fenestrae. In particular, the invention relates to the characterization of moesin as a component of fenestrae sieve plates. More particularly, the invention relates to the use of moesin as a plasma membrane marker. Moesin may be used as a plasma membrane marker for the identification of fenestrae or permeability of endothelial cells.