Just prior to uterus implantation mammalian blastocyst stage embryos have already started the process of generating and segregating different cell types required for continued development. Exactly how this remarkable transformation from a single fertilised egg to a highly ordered multi-cellular blastocyst occurs, within just a few days, remains a fascinating and enduring question. During this time embryonic cells either start to adopt specialised functions (differentiate) or they can remain in a so-called ‘pluripotent’ state, only differentiating later in development. Understanding the mechanisms behind such in vivo ‘cell-fate decisions’ is fundamentally important but can also provide insight into the proposed future application of stem cell based regenerative therapies. Research on mouse embryos has begun to address these cell-fate related issues and has highlighted the importance of tight temporal control of specific gene expression and the impact of relative cell position (i.e. on the outside of, or encapsulated within the embryo) to blastocyst formation. Herein, we propose to identify, verify and thoroughly characterise novel early mouse embryo cell-fate influencing genes. Candidate genes have already been identified by integrating our unpublished experimental data aimed at uncovering genes differentially expressed in outer and inner cells with pre-existing datasets describing gene expression throughout the whole of early development. Accordingly, we plan to disrupt the expression of these candidate genes within discreet cell populations of the developing mouse embryo and to observe whether this will affect their ultimate cell-fate. If so, we will determine how this impacts upon already known cell-fate related genes and mechanisms of cell segregation. We will also investigate the consequences of disrupting our validated/ novel cell-fate genes by assessing global effects on gene expression and in turn will yield additional candidate genes for future research.