Figure 1.

A skier's guide to negotiating Waddington's slopes. Self-renewing, tissue-specific stem cells might get back to the ‘pluripotency summit' by a single ride on Schöler's ‘OCT4 express' (Kim et al, 2009). If, however, you follow the slopes too far and a thrilling run takes you all the way to the ‘valley of terminal differentiation', you will definitely need the ‘four factor pass' to get back to the top. (Always read the label: reprogramming might not be complete and redifferentiation is subject to status; limitations apply and tumours might arise; the order of lifts on the map is approximate and might change according to conditions.) Compared with Waddington's bleak landscape, today's map provides a lot more information, but is still far from complete; other routes to pluripotency might exist, but skiers explore them at their own risk.