Fusion of the collagen type I alpha 1 (COL1A1) gene with the platelet-derived growth factor beta chain (PDGFB) gene has been described in dermatofibrosarcoma protuberans. The abnormal fusion transcripts probably cause PDGFB and its receptor (platelet-derived growth factor receptor beta, PDGFRB) autocrine stimulation and cell proliferation, which are responsible for the development of dermatofibrosarcoma protuberans. A reverse transcription-polymerase chain reaction assay was performed to detect the COL1A1-PDGFB fusion transcripts in 57 samples. In addition, the PDGFB gene amplification and PDGFB/PDGFRB mRNA levels were quantified by a real-time PCR system for the samples in which the fusion transcripts had been successfully detected. The fusion transcripts were detected in 42 of 57 samples. Various exons of the COL1A1 gene were fused in frame with the PDGFB gene; exons 7 and 25 were found to be slightly more frequently involved than the other exons. The PDGFB gene amplification levels varied from 0.6 to 8.3 (mean 2.4) in 42 tumor samples and from 0.4 to 3.0 (mean 1.2) in 20 adjacent normal tissue samples. In the 20 paired samples, the PDGFB gene amplification in the tumor was significantly higher than that in the normal tissue. The presence of PDGFB and PDGFRB mRNAs was demonstrated in 26 and 21 of 26 cases, respectively. The PDGFB and PDGFRB mRNA expression levels showed a good correlation (r=0.76, P<0.0001). These results indicate that the fusion protein, which is processed by the COL1A1-PDGFB transcripts, can serve as a functional ligand for PDGFRB.