Lmx1b is expressed in serotonergic neurons. Transverse sections through E11.5 pons (A, B), E12.5 caudal pons (C, D), E12.5 medulla (E, F), and E14.5 caudal pons (G-I) are shown. In situ hybridization was performed with the indicated probes (A-F). Lmx1b and Pet-1 are expressed in neurons derived from the ventral midline (A-F, arrows) that correspond to the primordial raphe nuclei. Lmx1b is also expressed weakly in the floor plate (A, arrowhead) and other areas in the medulla (A, star; E, arrowhead). G-H, Coronal sections through the E14.5 pons. Double immunostaining was performed with Lmx1b antibody (G, red) and 5-HT antibody (H, green). I is the superimposed image showing that Lmx1b is expressed in all 5-HT cells (with yellow nuclei) (I, arrow). A small subset of Lmx1b-positive cells is 5-HT negative (I, arrowhead).