Fig. 4. Helt determines the transmitter phenotype without affecting neuronal subtype identity specification. (A) Progenitor domain formation is not affected by the Helt-null mutation. Note that Nkx2.2 expression in the VZ is not changed by the Helt mutation, although postmitotic expression of this factor in the m2 domain is lost in the mutants. By contrast, expression of Nkx2.2 in m4 neurons is maintained in the mutants. (B) Conversion of the transmitter phenotype in Nkx2.2⁺ m4 neurons by the Helt-null mutation. In wild-type mouse embryos, most Nkx2.2⁺ neurons in the anterior mesencephalon express Gad1 but not Vglut2. By contrast, in the Helt mutants, most Nkx2.2⁺ neurons express Vglut2, although some populations maintain Gad1 expression.