The brain contains a huge number of neurons that have diverse characteristics participating in discrimination between individual neurons. It has been speculated that clustered protocadherins (Pcdhs), which encode cadherin-related transmembrane proteins as gene clusters in vertebrate genome, could provide these kinds of neuronal diversity. The murine Pcdhs are further classified into three subfamilies: Pcdh-α (14 genes), Pcdh-β (22 genes), and Pcdh-γ (22 genes). Their loss of function in mice revealed that the Pcdhs play important roles in neuronal survival, axonal projection, synaptic connectivity, and several brain functions including learning and memory. Similarly, recent works has implicated the Pcdhs in human psychiatric disorders. As revealed by histological examinations and single-cell RT-PCR, the Pcdhs show the scattered, probably the stochastic, expression in each cerebellar Purkinje cell. The scattered expressions of the Pcdhs will provide a potential neuronal diversity at the single-neuron level. The involvement of the scattered Pcdh expression in neural circuit formation has been inferred on the basis of several genetic analyses including loss of Pcdh-γ and loss of gene regulators of the Pcdhs (CTCF and Dnmt3b). However, several key questions remain unanswered. For example, are the Pcdh expressions scattered in other neuron type? Are the Pcdh expressions invariable in a live neuron? Are the Pcdh expressions truly stochastic? In order to answer these questions, we generated knock-in mice that harbor cDNA encoding red fluorescent protein, tdTomato, under the control of endogenous Pcdh-β3 promoter. The mice showed scattered tdTomato fluorescence in various neuron types, including cerebellar Purkinje cells, hippocampal CA1 pyramidal cells, dentate gyrus granule cells, cerebellar molecular layer interneurons, etc. The newly developed antibody against tdTomato enables a high signal-to-noise ratio detection of tdTomato. We are currently addressing the key questions about scattered Pcdh expression.