Fusion of the collagen type I α 1 (COL1A1) gene with the platelet-derived growth factor B-chain (PDGFB) gene has been described in dermatofibrosarcoma protuberans (DFSP). Various exons of the COL1A1 gene have been shown to be involved in the fusion with exon 2 of the PDGFB gene. We examined the breakpoints of the COL1A1 gene using the tumour specimens from four patients with DFSP. The COL1A1-PDGFB fusion transcripts were detected from the cultured tumour cells by reverse transcriptase polymerase chain reaction. Sequence analysis revealed that the ends of exons 23, 25, 26 and 36 in the COL1A1 gene were fused with the start of exon 2 in the PDGFB. This study identified three novel COL1A1 breakpoints: exons 23, 26 and 36 of the COL1A1 gene. In one case, the tumour was composed of two areas that differed in cytological atypia, cellularity and mitotic activity, indicating the dedifferentiation of the tumour. In tumour cells from two different areas the same aberrant fusion transcripts were identified. These results suggest that the dedifferentiation of tumour cells has nothing to do with the specific breakpoints of the COL1A1 gene, but depends on other unknown factors.