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Gastrointestinal Involvement in Systemic Mastocytosis (SM): Immunostaining with CD25 and CD117 is helpful in identifying neoplastic mast cells in gastrointestinal biopsies. CD25 positivity must be interpreted with caution since some lymphocytes also express this marker. Tryptase expression in mast cells in GI tract of SM patients is often variable and reduced (see next slide).