CDC42 and cystic kidney disease: Figure 4A illustrates apical membrane domain Par-6 immunoreactivity in proximal tubules and, less convincingly, in distal renal tubules and collecting ducts. The Par-6 positive cyst epithelium also exhibited exclusive apical membrane labelling (Figure 4B,C). Thin-walled cysts occasionally displayed basal membrane Par-6 immunostaining (not shown). CDC-42 immunoreactivity was observed in renal tubules corresponding to the apical plasma membrane domain (Figure 4D). A similar labelling pattern was observed in all immunoreactive renal cysts (Figure 4E,F).