What developmental processes are responsible for establishing the sensory preference of cortical neurons? In visual cortex, neurons belonging to a particular class (e.g. morphologically similar neurons in layer IV) are diverse in their functional properties, including receptive field size, and spatial and temporal frequency preference. For each neuron, these response properties are primarily defined by the pattern of inputs it receives. On one hand, these inputs could be selected by stochastic or activity-dependent processes during development. On the other hand, the pattern of inputs may be guided by molecular factors that ultimately determine functionally distinct neuronal subtypes. In other words, an apparently homogeneous cell type may in fact comprise several functionally distinct subclasses established by their patterns of gene expression.Specifically, we will test (i) whether a morphologically homogeneous population of excitatory neurons is diverse at the level of their individual transcriptional profiles, (ii) whether any transcriptional differences between individual neurons are related to their visual response properties, and (iii) whether their molecular identities persist in the absence of structured visual input. To this end, we will extract the mRNAs and determine the transcriptional profile of individual neurons whose visual response properties have been characterised in vivo, using methods developed in the host lab. Together, this proposal will attempt identify the contribution of molecular determinants to shaping connectivity and response properties of cortical excitatory neurons.