- Neuroblastoma cells produce transforming growth factors during exponential growth in a defined hormone-free medium. van Zoelen, E.J., Twardzik, D.R., van Oostwaard, T.M., van der Saag, P.T., de Laat, S.W., Todaro, G.J. Proc. Natl. Acad. Sci. U.S.A. (1984)