Three myoepitheliomas (MEOs) derived from the salivary glands were examined immunohistochemically. Proliferating cell nuclear antigen (PCNA)-positive cells were very rare (less than 2% of all tumor cells) in localized tumors of case 1 (epithelioid) (E-oid) cells) and case 2 (plasmacytoid) (P-toid) cells with a small number of spindle-shaped cells), but the percentage of PCNA-positive cells was high (21.8%) in case 3 (clear cells) exhibiting bone destruction. Strong c-myc expression was detected in all the tumors, but p53 or c-erbB-2 protein was not detected in any of the cases. More than half of the clear cells were positive for epidermal growth factor (EGF), while fewer tumor cells in cases 1 and 2 expressed EGF. A few tumor cells in cases 2 and 3 were positive for EGF-receptor (R). Keratin was most prominent in the E-oid cells, The P-toid cells were most strongly positive for S-100 protein followed by the E-oid and clear cells. More than half of the spindle-shaped cells and one-third of the E-oid cells were positive for alpha-smooth muscle actin (alpha-SMA), but less than 5% of the clear cells and none of the P-toid cells were positive for alpha-SMA. These results suggest that tumor cells in MEO are heterogenous and have different proliferation activities.

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