What do differentiated cells do with DNA they don't need?

Every cell in the entire organism contains all the DNA that the original zygote had. The genes that the specialized cells don't use are inactivated.

In 2006, Shinya Yamanaka made a groundbreaking discovery. He found a new way to reprogrammed adult, specialized cells to turn them into stem cells. These laboratory-grown stem cells are pluripotent- they can make any type of cell in the body - and are called induced pluripotent stem cells, or iPS cells.

Only embryonic stem cells are naturally pluripotent. Yamanaka's discovery means that theoretically any dividing cell of the body can now be turned into a pluripotent stem cell.