The expression of P-gp and S-100 in brain tumors (astrocytoma),(×400). (a, b, c) The expression of P-gp is weak in tumor cells (red arrow), but strongly positive in capillary vessels (black arrow). (c) The expression of P-gp in the interstitial cells was related to the distance from the capillary wall. The expression of P-gp was stronger the nearer the cell was to the capillary wall (green arrow). (d) The expression of S-100 in brain tumors. Our study shows the expression of P-gp and S-100 are co- localized in the capillary endothelial cells and interstitial cells of tumor tissues. These findings suggest P-gp expression at the microvasculature is found at both the endothelium as well at the astrocyte end-feet at the microvasculature.