Neurons proved to be a convenient experimental model for studies of regulatory properties of the genetic apparatus during development. Formation of neuronal ensembles is controlled by multiple modifier-genes at different levels. The following gene groups can be distinguished: 1) Genes that function in differentiating neurons; 2) Genes that function in other neurons; 3) Genes that function in glial cells; 4) Genes that function in surrounding tissues; 5) Genes that function at the organism level.