PCR analysis indicated that divergent primers could produce the circular isoform of MMP9 with cDNA but not with genomic DNA (gDNA), while convergent primers could amplify the linear isoform of MMP9 from both cDNA and gDNA in the 3 GBM tissues, U87 cells and U251 cells (Additional file 2: Figure S1C). This evidence concerns the gene MMP9 and glioblastoma.