To better understand the terms throughout this chapter, we provide a brief overview of the human genome. More comprehensive background can be found in the reference section.

The genetic information in our cells is coded by the DNA molecule (Figure 6-1), which is composed of four chemical building blocks, abbreviated by the letters A, C, G and T. Each cell contains two copies of each building block, one copy inherited from the mother and one from the father. This creates a three-billion letter DNA strand. The DNA molecules making up the strand are packaged into 23 pairs of very long pieces called chromosomes. Each of the 46 chromosomes contains genes, which are stretches of letters within the DNA that encode the instructions to make proteins. The DNA regions that do not contain genes are called non-coding regions. There are approximately 20,000 genes in the human genome, however, not all of them are active in every cell at any given time. Many factors determine whether a gene will be active or not in a particular cell, and whether its corresponding protein will be produced. For example, some non-coding regions control gene activity and can turn a gene on or off. In addition, a chemical modification around a gene or within it, called methylation, can alter the gene activity.

Once a gene becomes active its DNA letters are copied to another molecule called RNA in a process named transcription. Levels of RNA molecules from a given gene can be measured and indicate how active the gene is. Each RNA molecule is then further translated into a protein which is the main building block of cells.

Cancer cells arise from normal cells by modifying the structure and/or number of genes that control cell growth, cell death, ability to spread to other parts of the body, and other important processes that give the cancer cells survival advantage and makes them hard to destroy. The modified genes and their products, namely RNA and proteins, can be used as biomarkers in pediatric cancer as described below. Many of the genes involved in childhood cancer control the normal process of fetal development which make them quite distinct from adult cancers.