PDGFRA and neoplasm: Surprisingly, despite PDGFRα was prominently expressed in tumor OPCs and its activation has been believed to be essential for normal OPCs, PDGF alone was not sufficient to support the growth of tumor OPCs (Figure 2A,B; Figure S4A, Supporting Information), suggesting the importance of the proper function of IGF1R for the influx of growth signals mediated by multiple RTKs beyond IGF1R.