1. A researcher is examining the distribution of an ion channel protein in the kidney. She incubates slices of kidney tissue in a dilute solution of a specific antibody directed against the protein, then uses the immunoperoxidase method to localize the ion channel proteins. She notes the presence of brown pigment in a population of epithelial cells, which on closer examination, have a brush border. The researcher concludes that the protein is probably present in cells of the