At an elementary level, the development and homeostasis of an organism are the result of the differential expression of thousands of genes, the basic heredity units. Each gene consists of a portion of the DNA that contains sequences that code for a protein and noncoding sequences that determine its spatial and temporal expression pattern. The interregulations of a subset of an organism's genes form genetic hierarchies that dictate pathways that are responsible for the formation of the organism and its maintenance. The identification and analysis of genes in these networks allows scientists to understand the fundamental principles of development and identify how misexpression or mutations that eliminate gene function lead to genetic diseases.