An example of Cre expression in mouse forebrain circuits, with labeling of specific neuronal projection systems in the cerebral cortex and striatum. (a) The ETS domain transcription factor (etv1) BAC drives Cre recombinase expression in layer 5 corticostriatal nuerons. (b) The neurotensin receptor (ntsr1) BAC drives Cre expression in layer 6 corticothalamic neurons. The projection axons of these neurons, which terminate in the dorsal thalamic nuclei, are clearly labeled. In the striatum, the majority of neurons are medium spiny projection neurons, which are evenly divided into striatopallidal (indirect pathway) and striatonigral (direct pathway) neurons, which selectively express the dopamine receptors Drd2 and Drd1a, respectively. Cre expression produced in drd2 BAC-Cre lines (c) is directed to striatopallidal neurons. In this line, labeled neurons in the striatum extend axons that terminate in the globus pallidus external segment (GPe). (d) Expression produced in drd1a BAC-Cre lines, is directed to striatonigral neurons, which have axons that extend through the globus pallidus to terminate in the internal segment of the globus pallidus (GPi) and substantia nigra (not shown). Images courtesy of Charles Gerfen, National Institute of Mental Health.