Cell lineage decisions during development require strict gene regulation by complex networks of multiple transcription factors and collaborating factors. During fate decisions, key transcription factors (TFs) drive precursors intoone lineage while inactivating "alternative fate genes". Silenced states of a subset of "alternativ fate genes" are heritably maintained through epigenetic silencing. Epigenetic silencing is globally important for normal cell development and commonly altered in many types of cancers. Establishment of epigenetic silencing involves recruitment of enzyme activities mediating repressive histone methylation and DNA methylation. These enzyme activities are thought to be delivered by specific sets of TFs in a locus- specific manner. However, the mechanisms by which these TFs drive cel fate decisions and then establish epigenetic silencing are not completely understood. In this proposal, we will study crosstalk between key TFs driving T cell lineage choices and epigenetic modifiers that ultimately establish cell lineage identities. T lymphocyte development is an excellent system to tackle these questions because of our strong background knowledge on developmental processes and requirements for TFs. Specifically, we will study functions of Runx transcription factors in gene repression in the context of fate decisions and epigenetic silencing of "alternative fate genes". Our efforts will be focused on study of functional collaborations between Runx proteins and their interacting factors that drive and stabilize the fate choice processes. Because Runx proteins are involved in normal development of many cell types, and their functions are frequently altered in leukemia, knowledge obtained through our study of Runx-mediated genetic and epigenetic regulation during T cell development would provide substantial insights into stable gene regulation mechanisms widely used in many cell types as well as altered genetic programs causing cancers.