Translation of genetic information into proteins
With very few exceptions, every cell of every organism contains the complete genetic information for that organism, i.e. all genes are present in every cell. However, not all genes are needed at all times. The genes that are needed in a particular cell at a particular time are "switched on".
"Expression" occurs when a gene is "switched on" and is being used to make a protein. The tasks a cell performs depends on which of its genes are being expressed.
Characterising gene expression profiles makes up an important part of molecular biology. For transgenic plants, it is important to measure the expression levels of foreign genes. Transgene expression can vary significantly depending on how the foreign gene is integrated into the genome.