The differences in our genes come about through a natural process called mutation. Mutation is extremely common. As a cell copies its DNA before dividing, it makes occasional typos. Most commonly, a single base is substituted for another. Sometimes a base is deleted or an extra base is added. The cell is able to repair most of these typos. But each round of cell division introduces a few changes that are not repaired. When DNA changes occur in cells that will give rise to eggs or sperm, they can be passed down to offspring. Each one of us has about 60 new variations that were not present in our parents. People commonly use the terms mutant and mutation to describe something undesirable or broken. But mutation is not always bad. Most DNA changes fall in the large areas of the genome that sit between genes and they usually have no affect. When variations occur in areas of the genome that code for proteins, they often have consequences. These changes can effect the protein product itself or they can effect when, where, or how much protein is made. Most of the time, mutation generates variations that are neither good nor bad, just different.