Cancer is a complex disease, like diabetes, heart disease, and kidney disease. All complex diseases arise from combinations of changes that occur in the same cell over a period of time. In cancer, these changes accumulate over many years. Only when a critical number of changes occur in the same cell does it finally become cancerous.

To complicate matters further, there are many different types of cancer, each displaying different combinations of changes. Even within a single type, such as lung cancer or colon cancer, clinicians can identify subtypes, each marked by a unique set of changes.

How can researchers hope to identify and study all the changes that occur in so many different cancers?

How can they explain why some people respond to treatment and not others?

How can they explain why some smokers get lung cancer, and others do not?

How can these differences be explained?