and a following protein puriﬁcation conﬁrmed the presence of a secreted PDGF-like activity (18). Further puriﬁcation and characterization attributed this activity to a 31-kDa dimeric protein with struc- tural, functional, and immunological similarities to PDGF (19). At this time, Wong-Staal and co-workers had already reported a human gene homologous to v-sis, simian sarcoma virus (SSV) oncogene (20). But it was not until PDGF was sequenced in 1983 that the homology between the PDGF B-chain and p28v-sis was revealed and PDGFB (c-sis) was identiﬁed as a proto-oncogene (21). The cell surface receptor for PDGF, now known as the PDGF b-receptor, was found to be a tyrosine kinase (22). Similarly, EGFR was a tyrosine kinase, homologous to the v-erbB oncogene (23). These groundbreaking ﬁndings brought a lot of interest to the ﬁeld, which was spurred by the homology between p28v-sis and PDGF-B and by the fact that SSV originally had been isolated from a ﬁbrosarcoma in a woolly monkey (24). When tested on ﬁbroblasts and other normal cell types, the PDGF protein isolated from glioma- conditioned media showed receptor-binding charac- teristics and biological effects different from those of PDGF-BB and -AB. It inhibited PDGF-BB-induced chemotaxis and actin reorganization and had a lower mitogenic activity than PDGF-AB puriﬁed from pla- telets (25). These ﬁndings were initially challenged but could later be conﬁrmed by other groups and explained by the presence of a second receptor for PDGF. Further work identiﬁed the puriﬁed glioma- derived activity as PDGF-AA. Thereafter it was dis- covered that glioma cells produced PDGF-AA, -AB, and -BB dimeric proteins (26). Human PDGFA was cloned by Betsholtz et al. after generating a cDNA library from a clonal glioma cell line, U-343 MGa Cl2:6, that produced high amounts of PDGF receptor-competing activity. The isolated cDNA included two splice variants of PDGF-A, a short form (PDGF-AS) and a long form (PDGF-AL), which in the latter case includes a 69 base pair (bp) extension from exon 6, that codes for a C-terminal stretch of basic amino acids (13,27,28). While the PDGFB gene was localized on chromosome 22, the PDGFA gene was found to be residing on chromo- some 7. By sub-cloning cells from the U-343 MGa cell line, Nistér et al. identiﬁed an extensive clonal variation in the amount of PDGF secreted by glioma cells and also a clonal variation in the capacity of glioma cells to bind 125I-PDGF (29). The cells growth rate in serum- free medium correlated to the amount of PDGF pro- duced by them. The most prominent PDGF producers were also ‘immature’, tightly growing cells, while PDGF and PDGF receptors in glioma 101 clones with large, glia-like cells showed the highest 125I-PDGF-binding capacity. Thus, there was a neg- ative correlation between the levels of PDGF secreted from the cells and their capacity to bind the factor. By Northern blot analysis, using a PDGFR-b cDNA sequence as a probe, Nistér et al. identiﬁed PDGFR-b mRNA in some of the glioma cell lines and clones. Interestingly, the U-343 cell clones with the highest capacity to bind 125I-PDGF-AA displayed a different- sized PDGF receptor mRNA. Claesson-Welsh et al. were then able to identify the second PDGF receptor, PDGFR-a, expressed in U-343 MGa 31L cells (30). Further interest came when the PDGFR-a cDNA was cloned (31,32). Using 125I-binding assays, the pres- ence of both PDGF receptors and EGF receptors on human glioma cells was also described (33). With the new cDNA, mRNA, and antibody reagents available at Heldin’s laboratory that recog- nized PDGFs and their receptors, investigation into distribution was commenced using human brain tissue, in collaboration with Keiko Funa. Using mRNA in situ hybridization, high levels of both PDGF-B and PDGFR-b mRNA were found on the characteristic glioblastoma vasculature, while the glial tumor cells expressed more PDGF-A mRNA(34). This was conﬁrmed by a study of all glioma grades, where it was also observed that the PDGFR-a was present on glial tumor cells but not on the vasculature. Much to our excitement, it was shown that PDGFRA was overexpressed already in low-grade diffuse astro- cytomas and further enhanced in high-grade tumors. The ﬁndings indicated the presence of PDGF auto- crine/paracrine loops stimulating glioma cells via the a-receptor, and a PDGF-B autocrine loop stimulat- ing the vasculature via the b-receptor (35). PDGFRA expression was examined in a larger number of glioma samples of different malignancy grades (36). Increa- sed levels of PDGFR-a mRNA were observed in all grades, but glioblastoma expressed the highest levels. Only 16% of glioblastomas showed PDGFRA ampli- ﬁcation, which suggested that other mechanisms were responsible for PDGFRA overexpression in the majority of tumors. The latter has also been indicated by more recent studies (37,38). PDGFRA expression correlated to loss of heterozygosity (LOH) on chro- mosome 17p, where the TP53 gene is localized (36). PDGFRA ampliﬁcation had been described by Fleming et al. (39) and was conﬁrmed by Hermanson et al. (36). More recently it has been reported that PDGF-C and -D are also present in human glioma (40). The testing of the ﬁrst generation of TKIs, that were more or less speciﬁc for the PDGF receptors, was disappointing. The inhibitors did not signiﬁcantly alter the in vitro growth of glioma cells. Moreover,