If the final result is negative, e.g. no staining, then antigen retrieval is recommended. Keep a stock of 0.1% trypsin (Sigma type II, catalog no. T-8128) dissolved in 0.02 M PBS in frozen aliquots. For antigen retrieval, dilute the stock 1:100 to obtain a final trypsin concentration of 0.001%. Add CaCl2 to a final concentration of 0.001%. Incubate the sections in this solution for 1-2 min. at room temperature and then rinse twice in 0.02 M PBS. Add trypsin-inhibitor (Sigma type II-S, catalog no. T-9128) to the primary antibody solution. Keep a stock of 0.1% trypsin inhibitor diluted in 0.02 M PBS in frozen aliquots. Dilute the stock 1:100 into the primary antibody solution.