Dissecting inter-regional connectivity at the cellular level is a key step towards understanding the functional oper-ation scheme of a complex brain network.Large numbers of highly heterogeneous neurons are often packed within a small region.Traditional long-range projection tracing techniques mostly rely on the injection of viral or chemical tracers and the direct visualization of axon terminals at distant sites by optical imaging[1].The diverse projection patterns of individual neurons are often difficult to discern if many neurons in the same region are labeled in bulk with the same fluorescent or enzymatic label(Fig.1A).