The mechanisms that regulate transcription initiation and elongation by RNA polymerase II have been studied extensively because this polymerase is the one that transcribes mRNAs. Transcription initiation and elongation by RNA polymerase II are the initial biochemical processes required for the expression of protein-coding genes and are the steps in gene expression that are most frequently regulated to determine when and in which cells specific proteins are synthesized. As noted in the previous section, the expression of eukaryotic protein-coding genes is regulated by multiple protein-binding DNA sequences, generically referred to as transcription-control regions. These sequences include promoters, which determine where transcription of the DNA template begins, and other types of control elements located near transcription start sites, as well as sequences located far from the genes they regulate, called enhancers, which control the type of cell in which the gene is transcribed and how frequently it is transcribed. In this section, we take a closer look at the properties of various transcription-control elements found in eukaryotic protein-coding genes and some techniques used to identify them.