The cerebellum plays a major role in sensory motor adaptation and non-motor tasks. Initially, the cerebellar cortex was considered as a homogeneous structure with a “crystal-like” organization at the level of the cytoarchitecture and neuronal connectivity. However, it is now apparent that the cerebellar cortex is organized in modules found at the functional and molecular levels leading to a large diversity of information processing. A high heterogeneity in the neuronal populations forming this network is starting to be deciphered. But, while neurochemical and functional heterogeneity in Purkinje cells (PC) have been well described and its relationship to the diversity of functional properties for PCs is better understood, diversity of other neuronal populations in the cerebellar cortex is very poorly studied. Some markers have been found to be expressed heterogeneously in granule cells (GC), especially along the antero-posterior axis and single cell transcriptomic analysis has defined at least five types of GCs. The goal of this project is therefore to determine whether the molecular heterogeneity of GCs underlie a functional and computational heterogeneity in the cerebellar cortex. In task 1, we will assess the spatial and functional organization of two different GC lineages that are organized in two different antero-posterior gradients. Thanks to two genetically modified mouse lines already available and characterized as labeling these two GC lineages, we will describe their relationships to specific cerebellar modules defined by Purkinje cells markers, by combining anatomical, electrophysiological, optogenetic and imaging techniques. We will then determine the specific surfaceome for anterior versus posterior GCs using proteomic techniques and identify relevant markers for their specific connectivity and functional properties. Functional analysis of some of those markers will be performed using the Crispr/Cas9 technology to assess the relevance of GC molecular heterogeneity for their functional diversity. In task 2, using an intersectional approach, we will test how two parameters, GC genetic lineage and GC date of birth, interact and influence local molecular heterogeneity and functional synaptic properties. For this we will combine our two genetically modified mouse lines with electroporation of Cre dependent reporter constructs at different postnatal timepoints. Using sophisticated imaging and optogenetic techniques, we will describe how these two parameters interact to influence GC network integration and the diversity of their synaptic properties. In task 3, we will address the influence of the afferent mossy fibers on GC diversity, In a manner similar to how PC afferents influence PC diversity. We will study how specific mossy fibers target GC subgroups, defined by their lineage and the markers that will be identified in our work? We will also invalidate some markers with a potential role in regulating connectivity to test whether MF connectivity pattern is modified. Finally, we will test whether removal (using Diphteria toxin receptor expression) or activation (using chemogenetic tools) of mossy fibers during development modify GC diversity at the molecular and functional level. In task 4, we will assess how these results influence our existing models. Altogether our project will combine the complementary expertise of two groups internationally recognized for their work on the synaptic and functional organization of the cerebellar cortex to address key issues about the diversity of cerebellar synaptic organization and computation. The results will impact our understanding of brain development and function. Our results will also be of interest for the treatment of brain diseases since the cerebellum is also involved in several types of brain disorders such as autism spectrum disorders.