Regulation is the controlled expression of gene functions. This can be done in many ways, but these can be grouped into two classes. The level of enzyme activity can be regulated by noncovalent or covalent modification of a protein. The amount of the protein can also be regulated. This latter class of regulation can be exerted at any step in the pathway of gene expression or during protein turnover. For many (perhaps most) genes, the principal level of regulation of expression is at transcription, and Part Four of this course will focus primarily on this. However, post-transcriptional control is also important in many genes, and this will also be discussed.