Schematic of the Ras-MAPK and TGFβ pathways. The left-hand side of the schematic depicts the Ras-MAPK pathway while the right-hand side of the schematic shows the Smad-dependent TGFβ signaling pathway. The details of both pathways are elaborated in the text. Both pathways begin with extracellular factor binding transmembrane cell surface receptors and end with transcriptional changes. Differences in the Ras-MAPK and TGFβ pathways are evident in the method by which stimulus signals are relayed to the nucleus. The Ras-MAPK pathway utilizes a phosphorylation cascade that results in signal amplification and a nonlinear switch-like response to stimulus. The TGFβ pathway is depicted here as a linear response to stimulus due to the direct interaction of transcription factor Smads with transmembrane receptors TGFβRI and TGFβRII. Box 1, 2 and 3 indicates points of integration that are detailed in the text.