PDGFRB and gastrointestinal stromal tumor: An upregulation of phosphorylated PDGFRB (p-PDGFRB) was observed in NCC-DFSP4-C1 cells, which were resistant to imatinib, while a downregulation of p-PDGFRB was observed in GIST-T1 and NCC-DFSP3-C1 cells as sensitive cell lines.