AbstractST2‐3T3, a spontaneously transformed BALB/c‐3T3 cell line which does not require platelet‐derived growth factor (PDGF) for growth, was fused to THO2, a PDGF‐responsive non‐transformed BALB/c‐3T3 cell line, in order to learn whether transformation is expressed coordinately with PDGF independence. Hybrid cells were selected and grown in medium containing both HAT (hypoxanthine‐aminopterin‐thymidine) and ouabain; unfused cells of each parental type were killed in HAT‐ouabain medium. Five independently isolated ST2‐3T3xTHO2hybrid cell lines were established and characterized for both transformation and PDGF responsiveness. All five were transformed, having a disorganized growth pattern and achieving a final cell density similar to that of ST2‐3T3 cells. Two of these lines did not respond to a brief treatment with PDGF: the mitogen neither induced the synthesis of a PDGF‐modulated lysosomal protein (termed MEP), nor stimulated the cells to enter the S phase; one line responded to PDGF by synthesizing both MEP and DNA, whereas two others synthesized MEP but not DNA. In contrast, four independently isolated cell lines obtained by fusing PDGF‐responsive non‐transformed BALB/c‐3TC cells to the THO2line were all PDGF‐responsive for both MEP and DNA synthesis and were not transformed. It appears that PDGF independence is not required for the tra
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