While somatic cell reprogramming is now part of our text books, we ignore most of the mechanisms governing this cellular transformation. The most enigmatic question is why only rare cells undergo reprogramming, and whether this is governed by stochastc or deterministic events. In the late 2012, several major studies have addressed this question through a clonal analysis of the reprogramming process in murine MEF. In this mini-review, we describe these results and discuss future perspectives based on these date to optimize and secure the derivation of iPSC.