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A new strategy for visualizing neuronal cell morphology of mouse brain based on Cre/loxP recombination-activated gene expression is described. A "reporter" transgenic line was generated which expressed a fusion gene encoding a dendrite-targeted green fluorescent protein (MAP2-GFP) upon deletion of a transcription/translation STOP (transcription and translation stop signal) cassette. Cre transgenic "deleter" lines were established that activated reporter gene expression at various frequencies in pyramidal neurons in the forebrain. A deleter line was identified which activated a MAP2-GFP reporter gene at very low frequency (less than 0.1% of pyramidal neurons) and allowed the visualization of dendritic structures of individual neocortical and hippocampal pyramidal neurons. In addition, vertical "columns" of pyramidal neurons in the neocortex were labeled in these mice. In a second deleter line, a MAP2-GFP reporter gene was selectively activated in pyramidal neurons of the CA-1 subregion of the hippocampus in young mice. With its combinatorial property, this binary recombination-activated neuronal marker system should facilitate the study of detailed morphology, connectivity, and plasticity of defined classes of live neurons in vitro and in vivo., (Copyright 2002 Wiley-Liss, Inc.)