In previous studies we reported that the expression of HLA-DR on melanoma cell lines was differentially modulated by IFN- gamma and that the transcription rate was responsible for this differential modulation. We have also reported the nucleotide sequence variations in the promoter region of HLA-DR genes, and proposed that differences in the promoter activity by the sequence variations of the HLA-DR promoters might contribute to such a differential transcriptional regulation at the promoter level. In this study, in order to assess whether the sequence variations of the HLA-DR promoters affect the factor binding and exert influence on the promoter activity, nuclear factor binding to our previous six HLA-DRA and fourteen HLA-DRB promoter clones was evaluated with the nuclear protein extracted from a B-lymphoblastoid cell line (BLCL), BH, together with the chloramphenicol acetyltransferase (CAT) reporter assay. In the HLA-DRA promoters, clone #35 containing one bp nucleotide sequence variation at the octamer binding site (OCT) (GATTTGC to GATCTGC) showed relatively weak factor binding. In the HLA-DRB promoters, clusters I, III, and IV of our previous HLA-DRB promoter homologues, containing one bp nucleotide sequence variation (GATTCG) in their Y boxes exhibited weak factor binding and CAT activity compared to other clusters (GATTGG) that showed strong factor binding and CAT activity. This data suggests chat the binding patterns of transcription factors influenced by the nucleotide sequence variations of the HLA-DR promoter could affect the promoter activity and the DNA sequence elements in the HLA-DR promoter could mediate transcriptional regulation.