The modulation of cell physiology forms the basis of cellular growth and differentiation. Aberrant steps in this process are responsible for deregulated proliferation and constitute the molecular causes of tumorigenic development. Signal transduction events regulate gene expression by modulating the activity of transcription factors. Two principal pathways are used by the cell to convey signals to the nucleus. In one, cell surface receptors bind growth factors or peptide hormones and thereby activate intracellular signalling systems. In this case, transcription factors are targets for modification by transduction pathways. The other pathway involves nuclear receptors which bind to steroid and thyroid hormones, vitamin D and retinoic acid. In this case the nuclear receptors themselves are transcriptional regulators.