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T-cell populations of 22 plaque lesions from seven mycosis fungoides patients were studied for clonal rearrangement of the β chain of the T-cell receptor (Tβ gene. All plaque lesions employed in this study showed clinically similar appearance. Histologically, all the biopsy specimens showed epidermotropism and the dermal infiltration of mononuclear cells including atypical cells. Histochemically, the majority of the infiltrated cells had surface markers of helper T cells. DNA extracted from skin lesions, peripheral lymphocytes, and lymph nodes revealed that the monoclonal expansion of T cells was different among patients and lesions. DNA extracted from the two skin lesions of case 1 revealed a clonal expansion of T cells. The rearranged bands persisted for about 1 year. In contrast, all lesions from casts 3-7 showed no rearranged band. Interestingly, three lesions from case 2 showed mixed type results, i.e., the monoclonal expansion of T cells was detected in one lesion but not in the other two lesions. Time course study of case 2 revealed that the same rearranged hand became detectable in all three skin lesions and a lymph node about 1 year later. These results suggest that in plaque lesions of mycosis fungoides, there are various stages of detectable monoclonality of infiltrating cells, although the clinical appearance of the plaques was similar, and that the variety of monoclonality may reflect the long clinical course of this disease. [ABSTRACT FROM AUTHOR]