One of the key features of cellular differentiation programs is stability. Although differentiation is reversible in principle, many components of the gene batteries induced upon terminal differentiation are maintained throughout a cell's life. For example, muscle cells continuously express the myosin gene, and GABAergic neurons continuously express genes for GABA synthesis and transport. Maintaining gene expression patterns in the nervous system is a particular challenge given the non-renewing nature and therefore extensive life span of many neuronal cell types.