1. A 68-year-old man presents to his physician with complaints of fatigue and a dragging sensation in his abdomen. Physical examination is remarkable for massive splenomegaly without lymphadenopathy. The patient's hematocrit is 30%, his platelet count is 80,000/mm3, and his leukocyte count is 2500/mm3. Bone marrow biopsy shows "fried egg" cells expressing the pan B-cell markers CD19 and CD20, and the monocyte marker CD11c. Positive staining for which of the following substances would most likely verify the likely diagnosis?