Fig. 7. Further but incomplete rescue of Th expression at E17.5, and a schematic model of the expression and function of proneural bHLH in the development of midbrain DA neurons. (A-D) The number of Th⁺ neurons is further increased in Ngn2 and Ngn2^{KIMash1/KIMash1} mutants, but remains low at 9% in Ngn2;Mash1 double mutants. (E) We show that Ngn2 is expressed in VZ cells that express Sox2 and radial glia markers. Ngn2 is required for the differentiation of Sox2⁺ ventricular zone cells into Nurr1⁺ DA precursors, as deletion of Ngn2 results in a decrease in the number of Nurr1⁺ cells (step 1). This function is partially rescued by endogenous Mash1 recovered at E13.5 or by Mash1 expressed under the control of the Ngn2 promoter. Ngn2 is expressed by Nurr1⁺ cells in the IZ and a second possible function of Ngn2 is to promote the differentiation of Nurr1⁺ precursors into DA neurons (step 2). This step could be partially rescued by a substitution of Ngn2 by Mash1 (Ngn2^{KIMash1/KIMash1}). Thus, our results suggest that Mash1 can also partially rescue the number of DA neurons. Scale bar: 100 µm.