Neurogenetic analyses in the fruit fly Drosophila melanogaster revealed that gendered behaviors, including courtship, are underpinned by sexually dimorphic neural circuitries, whose development is directed in a sex-specific manner by transcription factor genes, fruitless (fru) and doublesex (dsx), two core members composing the sex-determination cascade. Via chromatin modification the Fru proteins translated specifically in the male nervous system lead the fru-expressing neurons to take on the male fate, as manifested by their male-specific survival or male-specific neurite formations. One such male-specific neuron group, P1, was shown to be activated when the male taps the female abdomen. Moreover, when artificially activated, P1 neurons are sufficient to induce the entire repertoire of the male courtship ritual. These studies provide a conceptual framework for understanding how the genetic code for innate behavior can be embodied in the neuronal substrate.