To understand the mechanisms of nuclear reprogramming and resistance to it, one needs to be able to judge the efficiencies of the various procedures; that is, the proportion of the starting cell population that responds to a reprogramming condition. If this proportion is very small, and if those cells that respond cannot be distinguished from those that do not, it is very hard to identify reprogramming factors and mechanisms. This is because most cells may not undergo reprogramming. There are striking differences in the speed and efficiency of reprogramming by different procedures and in resistance to it. There are two kinds of evidence for resistance to reprogramming. One comes from comparing nuclei from more or less differentiated cells; the other from comparing nuclei of different cell types. The efficiency of, and resistance to, nuclear reprogramming can be measured by many criteria. We have previously reviewed the criteria that can be used to judge reprogramming efficiency elsewhere [14]. Here, we only use the formation of different cell types or transcription of pluripotency genes as criteria (Figure 2).
