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Functional Characterization of the P53 Family Protein P63 and the EPHA2 Receptor Tyrosine Kinase, a Novel P53 Family Target GeneA cell’s ability to survive as part of a multicellular organism often depends on how well it communicates with other cells in its surrounding environment. The need for coordinated and complementary cellular activities requires a means o f sending and receiving signals to and from neighboring cells. This is achieved by the presence of an assortment of proteins both linked to and within the plasma membrane. The plasma membrane acts as a barrier between the intracellular and extracellular milieu, and plasma membrane proteins allow for much o f the communication between these two environments. An important class o f proteins found on the plasma membrane is the superfamily of receptor protein tyrosine kinases (RPTKs). RPTKs are membrane-spanning proteins, that contain domains on both sides of the plasma membrane, joined by a single transmembrane domain (1). Binding of a growth factor or cytokine to the extracellular domain of an RPTK results in receptor dimerization and autophosphorylation o f specific tyrosine residues on the cytosolic domains (1). These phosphotyrosine residues then act as docking sites for cytosolic proteins containing modular structures such as Srchomology- 2 (SH2) and phosphotyrosine binding (PTB) domains (2). Binding of SH2- and PTB-containing proteins initiates an intracellular signaling cascade that often results in the regulation of transcription factors in the nucleus (3-5). It is through these RPTKmediated cascades that many extracellular signals are received by a cell.