- Yap is a small protein that binds to many transcription factors and modulates their activity. Yap was described to increase the ability of p73 in inducing apoptosis as a consequence of damage to the DNA, and therefore its activity was thought to favor tumor-suppression. However, other studies have recently shown a role for Yap in cell differentiation, cell transformation and in the regulation of organ size. It has been demonstrated that the Drosophila Hippo pathway has a mammalian equivalent, and that Yap is part of this pathway, where it might stimulate proliferation. In light of these new findings we ought to re-consider the role of Yap, which seems to be in service of several masters, and whose regulation--likely at the level of PTM--and cellular context might have a pivotal role in the choice of its partners and consequently on the final outcome.