The Chargaff's rules indicate that in the DNA of all living organisms the amount of adenine (A) should be equal to the amount of thymine (T), while the amount of guanine (G) should be equal to the amount of cytosine (C). These rules were useful to describe the structure of the double helix and also enable to describe base patterns that often are associated with specific gene regions (for example, promoter regions are rich in AT). Moreover, an imbalance in the 1:1 equilibrium ratio is associated with mutations that may cause diseases such as cancer.
