Abstract Reversible and dynamic transitions between epithelial and mesenchymal cellular states (EMT/MET) contribute to cancer progression and dissemination. Whereas EMT facilitates initial steps of tumour cell detachment, MET is likely required for colonization at distant sites. Although MET is generally perceived as mirroring EMT, we hypothesize that MET entails its own set of novel and/or differentially active molecular circuitries, generating cells with features distinct from the original epithelial state. Using an in vitro TGFβ1-induced EMT/MET model, we demonstrated that MET generates co-existing heterogeneous cell populations (Reverted-Epithelial or RE-cells) with novel phenotypic and functional properties, such as increased self-renewal, in vivo increased tumourigenicity and distinct chemoresistance properties. Overall, our results indicate that MET is a permissive process, driving cellular plasticity towards heterogeneity and with it, creating novel biological signatures of relevance for cancer growth. Citation Format: Patricia Oliveira, Joana Carvalho, Sara Rocha, Mafalda Azevedo, Andre F. Vieira, Daniel Ferreira, Nuno Mendes, Ines Reis, Joao Vinagre, Alireza Heravi-Moussavi, Joana B. Nunes, Jorge Lima, Valdemar Máximo, Angela Burleigh, Calvin Roskelley, Joana Paredes, Fatima Carneiro, David Huntsman, Carla Oliveira. Epithelial-mesenchymal-epithelial transition induced by long term exposure to TGFB1 creates cellular heterogeneity. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1611.