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A knock-in mutation of Mash1 in the Ngn2 locus demonstrates that Mash1 is a determinant of V2 interneuron identity. Shown here is the ventral spinal cord of a mouse embryo homozygous for the Ngn2^KIMash1 allele, stained for the motor neuron marker Islet1 (red) and the V2 interneuron marker Chx10 (green). The substitution of Ngn2 by Mash1 in motor neuron progenitors results in the respecification of a subset of motor neurons into Chx10+, Isl1 V2 interneurons, while the differentiation of other motor neurons is not affected. (For details, see Parras et al., p. 324.)