A stem cell divides in a special way to produce two cells: one that is a copy of itself, and one that develops into a more specialised sort of cell. This more specialised cell then gives rise to other cells that are more specialised still, a process known as differentiation. So a cell’s future career options become increasingly restricted until you end up with a final, specific cell type.

Not all stem cells are created equal. Those renewing tissues in our bodies, like skin or liver, only seem to be able to produce a small range of cell types. These "adult" stem cells are at the centre of much controversy as to whether they can be made to form stem cells that are far more flexible and which can produce many different cell types.

The more extreme kind of flexibility is the hallmark of embryonic stem cells: cells from embryos that are only a few days old. The embryo is a ball of a few hundred cells and those destined to form its body are very flexible indeed. Pluripotency, or the ability to make virtually every cell in the body, makes ES cells extremely attractive to scientists working on regenerative treatments. Stem cells could, for example, renew tissue damaged by a heart attack, or replace brain cells lost in patients with Parkinson’s disease.

Making this work reliably and safely means knowing more about how stem cells work and how cells differentiate. Ultimately, this boils down to how the genes in any individual cells are deployed. Each type of cell in our bodies (except red blood cells, which lose most of their DNA) contains exactly the same genetic information. How can you get so many dif-ferent cell types if they all contain the same genes? The answer lies in how these genes are used during the process of differentiation.