Basic FGF (bFGF) and acidic FGF (aFGF) are multipotential factors that stimulate and support proliferation, migration and differentiation. Both bFGF and aFGF are non-secreted growth factors consistent with the lack of a signal peptide. However, bFGF and aFGF are deposited in extracellular matrix (ECM) suggesting that an alternative mechanism for FGF release exists. Four oncogenes, int-2, hst/K-fgf, FGF-5 and FGF-6 have been isolated that are highly homologous to aFGF and bFGF. Unlike bFGF and aFGF, they possess signal peptides and are secreted. These oncogenes transform cells and induce tumors, ostensibly via an autocrine mechanism. The involvement of bFGF and aFGF in autocrine transformation has been clarified by studies using FGF cDNA transfection. NIH-3T3 cells transfected with native bFGF cDNA and expressing 20 to 100 times as much bFGF as parental 3T3 cells acquire an enhanced proliferation rate and higher saturation density. NIH cells transfected with a construct in which bFGF cDNA is altered by addition of a signal peptide, undergo autocrine transformation and exhibit morphological and biochemical alterations characteristic of highly transformed cells. Injection of cells expressing native bFGF even at levels 100 times greater than parental 3T3 cells fails to induce tumors or lung metastasis in syngeneic mice. Signal peptide bFGF-transected cells on the other hand, acquire a high tumorigenic and metastatic potential with tumor incidence and numbers comparable to those induced by ras transformed cells. Acquisition of a signal peptide converts bFGF into a transforming protein analogous to FGF-related oncogenes which naturally have signal peptide sequences.