The 5' upstream regulatory region of the mouse tyrosinase gene contains a long (GA)n sequence, that may be capable of adopting a triple-helical conformation (triplex). We analyzed protein-DNA interactions in a part of the 5' upstream region containing the (GA)n sequence by gel retardation analysis and found evidence for a cell type-specific protein(s) that bound to this region. We also found a (TC)10 sequence about 100 bp downstream from a polyadenylation site of the gene. Examination of tyrosinase cDNAs and Northern analysis indicated that this sequence is transcribed and removed during 3' end-processing of the mRNA. Based on the hypothesis that the (TC)10 sequence binds to the (GA)n sequence and forms an intermolecular triplex, we performed the same gel retardation assay in the presence of the 3' non-coding RNA fragments containing the (UC)10 sequence. The probe DNA failed to interact with the cell type-specific protein(s). These results suggest a novel hypothesis for the regulation of the mouse tyrosinase gene, i.e. that the 3' non-coding RNA fragments of mouse tyrosinase transcripts suppress its own expression at the transcriptional level. This might occur by preventing cell type-specific protein factor(s) from binding to the regulatory cis-elements in the 5' upstream region of the gene, possibly through a triplex formation, although this hypothesis remains to be proven.