Diaminobenzidine (DAB) in situ hybridization revealed separate distributions of Lmx1b mRNA (a–d) and Foxp2 mRNA (e–h) expression across four rostral-to-caudal sections through the PB region in mice. Approximate level caudal to bregma is shown at the bottom-right of each panel (in mm). Translucent highlights identify the scp, brainstem surface, and fourth ventricular surface. Arrowheads in panels (b, c, f, g) highlight dense Lmx1b and absent Foxp2 in a region of the mouse PB that is homologous to the “external lateral” subnucleus in rats (Fulwiler & Saper, 1984). Scale bars in (a) and (e) are 200 μm and apply to all panels below. Other abbreviations: Cb, cerebellum; LC, locus coeruleus.