Mucosal associated invariant T cells (MAIT cells) are selected by MR1 expressing double positive thymocytes. In mice, the associated SLAM interactions lead to the expression of the transcription factor PLZF characterizing MAIT cells. MAIT cells undergo subsequent differentiation into MAIT1 (T-bet+) or MAIT17 (RORγt+)1,2 subsets. The molecular mechanisms are poorly known. In our previous scRNAseq analysis, the branching point between the two fates was located in an "intermediary" cluster in which it was difficult to delineate bona fide MAIT from mainstream-like precursors1. Recently, we found that the characteristics of the TCR are not involved in this MAIT1/MAIT17 commitment (under revision3). Gene expression analysis only reflects the RNA content at a given time. In contrast, the profile of accessible chromatin regions provides a view of the loci that are not actively transcribed but in euchromatin, revealing cell type-specific regulatory DNA elements and transcription factors (TFs) important for lineage determination. To better understand the underlying regulatory networks involved in MAIT cell states, we simultaneously profiled snRNA-seq and scATAC-seq in thymic MAIT cells from chimeras made of MR1wt B6-MAITcast BM donor into MR1-/-CD3e-/- hosts. Two strategies were tested: (i) comparison of the chromatin architecture of the “intermediary” vs MAIT1 or MAIT17 clusters previously defined by their transcriptional program and (ii) a sub-clustering of the “intermediary” cluster based on its chromatin structure. These analyzes will provide a comprehensive and dynamic landscape of chromatin accessibility and the first multi-omics analysis of the developmental trajectory of MAIT cells. As the epigenetic landscape is more stable than the transcriptome, we expect that the chromatin architecture in early MAIT precursors will identify new cytokines/TFs involved in MAIT development and better define the establishment of the differentiation program of MAIT cells.