To transmit the information encoded in the genome (DNA), genes need to be first copied (transcribed) into RNA and then translated into protein (such as messenger RNA, or mRNA for genes that code for proteins). The level of specific gene activity, or gene expression, is regulated by numerous factors in cells.

Similarly, the levels of non-protein coding RNAs such as microRNAs are also determined by the state of the cells and their activity.

See additional information on gene expression in this Nature documentary.