The neocortex consists of billions of neurons with trillions of connections, and is responsible for cognitive tasks such as perception and abstract reasoning.  Cortical abnormalities are associated with devastating neuropsychiatric illnesses including autism spectrum disorders and schizophrenia, further underscoring the importance of understanding how the cortex functions.  Despite major advances in understanding the properties of individual neurons, we are just beginning to understand the rules that govern the assembly of neocortical circuits.  One major unanswered question in the field is whether molecular mechanisms direct the wiring diagram of neural circuits at the level of individual cells.  In other words, is there a genetic blue print that governs the local wiring of cortical circuits?  Tis question is technically challenging to address because it will require simultaneous analysis of both neuronal connectivity and the gene expression profile at the level of single cortical neurons.  Moreover, it will require massive amounts of data to be collected as well as advanced analysis techniques to crack the molecular code of connectivity.  We propose to use multi-whole-cell patch clamp recording combined with single-cell reverse transcription (RT)-PCR to investigate whether the clustered protocadherin gene family provides a molecular code for neural circuit assembly.  The clustered protocadherins have been shown to be expressed in a combinatorial code in individual neurons, are highly expressed during synaptogenesis, and localize to synapses.  Thus, clustered protocadherins are the leading candidate molecules for shaping neocortical circuits.  However, their role in instructing specific synapse formation has not been directly tested due to the difficulty of these types of experiments.  Furthermore, the challenging aspects of this project make it unlikely to be funded by more traditional mechanisms.  The results of these experiments could revolutionize our understanding of neural circuit assembly and lead to a more principled approach to studying neuropsychiatric disorders at the circuit level.