ES cells possess a unique cell cycle quite different from committed cells, especially G1 is shortened and Rb is hyperphosphorylated during the whole cell cycle, indicating that Ncl should not be influenced or in a complex with Rb in ES cells. Ncl is also described to interact with or be part of several transcription factor complexes, one of them being the B cellspecific transcription factor and switch region binding protein, LR1. Ncl has been shown to activate endogenous Bcl-2 and CD34 gene expression in CD34 positive hematopoietic cells by a direct sequence-specific interaction with the CD34 promoter region and is thus involved in the maintenance of these progenitor/stem cells in hematopoiesis. Even though Ncl is expressed at high levels in ES cells, not much is known about its specific role or physical interaction network in this cell type; therefore we explored Ncl in ES cells, starting with a search for new interaction partners. In current study, we show that Ncl interacts individually with translationally controlled tumor protein and Oct4 in a cell cycle dependent manner, and both Dasatinib complexes require phosphorylation of Ncl. The data presented here reveals new protein-protein complexes involving Ncl in murine and human ES cells, where we have identified Oct4 and Tpt1 as two endogenous Ncl-P interaction partners. All experiments with murine ES cells were performed on two different ES cell lines in parallel. We found no discernible differences between the ES cell lines, and one experiment also included human ES cells, indicating our findings to be generally applicable for ES cells, and not species or cell line specific. In our previous search for Tpt1 interaction partners in ES cells we found that Tpt1 forms a complex with Npm1. In the same screen additional potential interaction partners were found. One of the factors found were actin, which has recently been reported to interact with Tpt1 and we have now also revealed that NclP forms a complex with Tpt1 in murine ES cells. Ncl has previously been reported to be expressed at high levels in both murine and human ES cells where high levels of Tpt1 also were reported. Recent proteome analysis of mouse ES cell lines has furthermore revealed that the presence of Tpt1 is a characteristic of undifferentiated ES cells. Tpt1 is well conserved and expressed in all eukaryotic organisms but no main function has yet been found. Accumulating evidence demonstrate that Tpt1 and Ncl are important for cell proliferation. Tpt1 is up-regulated during the entry into the cell cycle. Over-expression of Tpt1 results in slow growing cells and a delayed cell cycle progression whereas over-expression of a Tpt1 double mutant, in which the two Plk1 phosphorylation sites have been substituted for alanines.