Growth factor receptors (GFRs) are often aberrantly expressed in tumor cells,and altered GFR expression and activity contribute to the pathogenesis of many types of cancer.A variety of mechanisms have been identified that result in enhanced GFR expression and activity in cancer cells.Defects in the pathways responsible for GFR intealization and intracellular trafficking are likely to be involved in altered GFR expression in a variety of cancers.The roles of GFR trafficking pathways in the regulation of GFR expression,in the pathogenesis of tumors,and in the response of tumors to treatment have not been fully delineated,but the likely contributions of GFR signaling to the development and progression of various malignancies suggest that therapies that modify GFR trafficking may be effective as anticancer treatments.The intracellular trafficking of GFRs is regulated by a number of protein complexes and by protein ubiquitination.Many of the proteins required for this trafficking are products of tumor suppressor genes,and the expression and function of the protein machinery utilized for intracellular trafficking is frequently altered in tumor cells,consistent with the likely role of GFR trafficking in tumorigenesis.Many of the proteins involved in GFR trafficking have been identified as potential targets for anticancer treatment,and novel treatments directed against these targets are currently in preclinical development and in clinical trials.Ubiquitin ligases are critical for GFR trafficking and represent potentially important targets for the development of novel therapies.The genes for the ubiquitin ligases c-Cbl and UBE4B are located in chromosome regions commonly altered in a variety of tumors and therefore are likely to be important for tumorigenesis.c-Cbl ubiquitinates a number of GFRs and directs them for degradation.Mutations in c-Cbl have been identified in cases of myeloid leukemia and myelodysplasia,providing a link between GFR ubiquitination and trafficking and leukemogenesis.We have shown that UBE4B plays a crucial role in GFR trafficking and degradation in tumor cells,suggesting a previously uncharacterized link between UBE4B and tumorigenesis.With the critical need for new and effective therapies for pediatric malignancies,the recently Identified roles for the GFR trafficking pathway in the pathogenesis of various forms of cancer confirm the importance of the further development of novel therapies targeting this pathway in children with cancer.