In a cell, a gene is expressed when information is taken from the gene and used to create a protein. DNA transcription produces RNA, then RNA translation makes proteins. A cell expresses only a selection of the genes it contains at any one time. By controlling which genes are expressed, the cell controls its size, shape, and functions.
Proteins are the "workhorse" molecules, taking part in every structure and activity of life. To perform their functions, proteins communicate in a network, comparable to how humans behave in social media networks. In cancer, there are changes in proteins that lead to miscommunications.
Cells from healthy breast tissue and tumors have different characteristics . Once a genome has been sequenced, we know what potential a cell has—what characteristics and functions it might exhibit—based on the genes it contains. However, sequencing the genome does not tell us which genes a cell is expressing. Knowing which genes are expressed—the cell’s gene expression profile—gives us clues about the cell’s functions.
In gene expression profiling experiments, measuring mRNA levels shows which genes have been transcribed to mRNA and, thus, have the potential to be translated to protein. Changes in protein function can be detected indirectly by measuring the levels of the mRNA molecules that code for the proteins.