Publisher Summary This chapter discusses the method for studying the processing of amyloid protein precursor (APP) of Alzheimer's disease that is transfection of HeLa cells with an amyloid protein precursor expression plasmid. The expression and secretion of APP 695 from transfected HeLa cells can be monitored both by immunoprecipitation and by Western blotting. The method can be usefully adapted to studying a range of different aspects of APP processing and secretion. The monoclonal antibody (clone 22C11, which can be obtained from Boehringer Mannheim, Indianapolis, IN) used in this procedures recognizes a domain close to the N terminus. With the identification of other members of a now-expanding APP gene family, it is possible that the 22C11 antibody may recognize other APP-like proteins. Thus, the specificity of the antibody must be demonstrated in each tissue under examination. Studying the processing of APP in cells transfected with an APP cDNA expression plasmid eliminates this problem.