The human embryo before birth is made up of only a handful of cells. These cells divide and multiply in the small embryo and gradually become more specialised until they form structures such as bone and the heart.

After this, each cell divides and produces more of its own kind in a controlled way until the baby is born. The process of cells dividing and multiplying continues throughout life in order to replace old cells that have been damaged through wear or injury. Some cells such as skin and bone marrow can do this very quickly; others such as nerve and muscle do it slowly.

Sometimes things can go wrong You can see that the making of a human being is very complex and it is not surprising that things can go wrong from time to time.

Cancer develops when a cell in the body, for some as yet unknown reason, starts to divide and multiply without the normal control. A mistake in the cancer cell’s genes tells the cell to grow, divide and multiply. Millions of cancer cells can grow from one cancer cell.

At first nothing may be noticed, but a build up of cancer cells may produce a lump or tumour. Some types of cancer may produce other symptoms such as bruising, tiredness and aching joints. It is not known when a cancer really begins, but there is always a period of time before it becomes noticed.

Different kinds of cancer have different signs, symptoms, treatments, and outcomes, depending on the type of cell involved and the degree of uncontrolled cell growth. The most common childhood cancers are leukaemia, lymphoma, and brain cancer.