The chemical sequence of A's, C's, T's, and G's in the coding region of a gene determine the composition of the resulting protein and thus its function. The regulatory elements of a gene, also composed of a sequence of A, C, T, and G, are commonly upstream of the coding sequence but may also be interspersed within the coding sequence or downstream of it. Regulatory elements control in which cell types specific genes are activated, and the timing and magnitude of gene expression. Gene regulation thereby allows individual proteins to be expressed only in certain cells at specific times and at specific rates.