A 55 year-old man from Chicago complains of pain in his left thigh for five months. On physical examination his thigh is increased in size, compared to the right. A plain film radiograph reveals the presence of a 15 cm solid mass that does not appear to arise from bone, but it does have infiltrative margins. A biopsy of this mass is taken, and on microscopic examination the mass is composed of highly pleomorphic spindle cells. Which of the following markers is most likely to be demonstrated in the cells of this mass?