Foci are cells located in a specific organ of the body that are notably different from the surrounding cells. These differences are caused by mutation or other types of cellular damage, and they're generally the first sign of a developing lesion, tumor or other disease.
About 20 unique mutations to foci cells have been identified by scientists. Alterations to the cells can occur in the cell wall or in the organelles inside the cell. The cells may change color, shape or size as they begin to mutate. Some common places for foci cells to occur is in the liver, lungs and glands.