In the absence of its cognate ligand (retinoic acid, ATRA), retinoic acid receptor alpha (RARα) acts as a transcriptional repressor by recruiting corepressor (CoR) complexes to target genes. This constitutive repression is crucial in metazoans. However, its specific molecular determinants remain obscure. Acute promyelocytic leukemias (APLs) arise from different chromosomal translocations that create x-RARα fusion proteins (like PML-RARα and PLZF-RARα). Unlike RARα, x-RARα fails to release properly CoRs in response to ATRA and block differentiation of myeloid cells. There is a great need to decipher the interaction complexity within the RARα repressive complex, since disruption of this system shifts the balance toward abnormal gene regulation and pathology. The project RepreX proposes a novel approach to study pathogenic disturbance of RARα signaling, at the atomic, molecular and cellular scales. Its strength resides in the synergism that will emerge from an integrative approach combining structural (by X-ray crystallography and cryoEM) and dynamics studies (by SAXS, mass spectrometry (HDX-MS), and fluorescence methods (smFRET)) of the native full-length proteins and their functional complexes, and allowing to connect detailed structural models with biological function. Understanding the precise molecular underpinnings of RARα function requires integrative modelling of the flexible multi-domain and functionally dimerized RARα/RXR heterodimer bound to DNA and to intrinsically disordered transcriptional coregulators (like CoR). We aim at obtaining a complete structural and dynamic picture of the full repressive RARα/RXR complexes, in its physiological form (wild type RARα) and in its pathological form (PML-RARα and PLZF-RARα). These results promise to explain how signals coming from the various components are integrated and turned into a particular physiological or pathological response, providing better frameworks for guiding future drug discovery efforts. The structural information gained from this work will most likely reveal unknown specific binding sites and interaction surfaces representing unexplored opportunities for the development of novel targeted therapeutic strategies. In addition to improving our knowledge on retinoic acid signaling, the project RepreX will have important long-term repercussions for human health, regarding the great expectations of biologists and medical researchers for studies like it that will give a precise description of the molecular mechanisms of action of the x-RARα fusion proteins. We have already established collaborations with the group of H. de Thé (Collège de France, Hôpital Saint Louis) who will bring complementary knowledge on biology of PML-RARα and with the group of A. Yunes (Boldrini Hospital, Campinas, Brasil) whose clinical research is focused on elucidating the genetic and molecular mechanisms of leukemia progression and resistance to therapy.