All of us begin life as a single cell. This cell goes on to divide countless times to produce the trillions of cell that make up an adult human being. Even after we reach adulthood, and stop growing, many of the cells in our body continue to multiply in order to produce new cells to replace old cells that are worn out, and die off.

This process of cell division, and multiplication, is very carefully controlled so that exactly the right number of cells are being produced at any one time. We do not fully understand how these controls work but at the level of individual cells the process of division is triggered by growth factors.

Growth factors are proteins that fit into special sites on the surface of the cell, called receptors. When the growth factor binds to its receptor this sends a chemical signal into the cell. The signal activates another type of protein, called gene regulatory protein. This in turn triggers off the genes which control cell division, switching them on so that the process of reproducing the cell is set in motion.

In recent years many different growth factors have been discovered, and it is likely that most human cells need to be stimulated by a combination of several different growth factors, binding to its receptors, in order to start the process of cell division.

In some types of cancer we now know that there are chemicals which can act as growth factors and encourage the growth of the tumour. For example, some, but not all, breast cancers contain cells that have oestrogen receptors on their surface. If the female hormone, oestrogen, binds to these receptors, this will encourage the cells to multiply. So the oestrogen acts as a growth factor for these cells. Reducing the level of oestrogen in the blood (by suppressing the ovaries) or interfering with the oestrogen receptors (with drugs like tamoxifen) can prevent the stimulation of the cells and help control the cancer. In a similar way many prostate cancers have receptors for the male hormone, androgen, which can act as a growth factor for these tumours.