The Notch signaling system is a cell-to-cell communication mechanism that is found in virtually all multicellular organisms and allows developing cells to acquire their specialized cellular types (neuronal, muscular, digestive, reproductive, etc.). This system has many components, two of which are the main focus of this proposal. These are the Notch signal receptor molecule and the Serrate ligand that is capable of interacting with and affecting Notch receptor activity. The Serrate ligand can activate the receptor when these molecules are expressed on adjacent cells leading to the formation of different cell types. Serrate can also inhibit the receptor when these molecules are co-expressed on the same cell, preventing such cells from receiving a Notch signal and forming distinctive cellular types. This proposal investigates the specific regions and properties of the Serrate ligand that are responsible for inhibiting the Notch receptor hence investigating a crucial mechanistic aspect for controlling cellular specialization. The work utilizes targeted molecular-genetic mutation of the Serrate molecule with subsequent expression of these mutated forms in the fruit fly to investigate their effects on Notch signaling. The outcomes are expected to fully define and characterize the regions of Serrate that confer the inhibitory property of the ligand onto the Notch receptor and potentially define regions essential for Notch activation. The evolutionarily conserved nature of Notch system components allows for very broad applicability of the findings of this study to the homologous molecules in vertebrates. Further, this study will extend the understanding of cellular differentiation control in multicellular animals including stem cell differentiation mechanisms. The study will be conducted by undergraduate researchers providing these students with a strong background in molecular and genetic tools for subsequent professional training and development.