Extreme care should be used to identify and verify positive reactions, however, because cross-reactions are common. Counterstaining is essential for examining worms by immunofluorescence and is used to identify the exact cell in which an antigen appears. Methods for counterstaining include labeling all cells with a fluorescent dye that is specific for nucleic acids (e.g., DAPI or propidium iodide) and using GFP driven by tissue-specific promoters.