Transforming growth factor β (TGFβ) is an important cytokine that regulates cell growth, apoptosis and inflammation. It is secreted as part of a latent complex containing mature TGFβ, its cleaved propeptide and latent-TGFβ-binding protein (LTBP). This complex is stored in the extracellular matrix (ECM), releasing TGF-β in response to changes in the ECM. On p. 217, Justin Annes and co-workers present a novel hypothesis in which the latent TGF-β complex is viewed as a molecular sensor. Within this sensor, the propeptide functions as a `detector' that senses perturbations in the ECM by interacting with TGFβ activators such as integrins and thrombospondin and responds by releasing the `effector', mature TGFβ. LTBP acts as the `localizer' of the sensor, binding covalently to the ECM. The new model is consistent with the phenotypes of individuals in whom latent TGFβ processing or assembly is defective and explains several puzzling aspects of TGFβ biology. For example, the fact that TGFβ1, TGFβ2 and TGFβ3 have similar properties and expression patterns but isoform-specific effects in vivo can now be accounted for by differences in how their detectors respond.