The retinoblastoma (Rb) tumor suppressor pathway is mutated in most, if not all, human cancers. This pathway regulates genes required for S phase progression as well as genes essential for cellular differentiation, and has also been implicated in the establishment and maintenance of heterochromatin. Biochemical studies of the Rb pathway in vertebrates have been hampered by difficulties in extracting Rb complexes from cell nuclei in their native state. Genetic studies of the Rb pathway in vertebrates have been complicated by redundancy, long generation times, and the high cost of genetic screens. Fortunately, the Rb pathway has been conserved during metazoan evolution, permitting the informative analysis of simpler organisms. This proposal focuses on a recently discovered and quite unexpected connection between the Rb tumor suppressor pathway and the Myb oncogene family. Fractionation of nuclear extracts from Drosophila cells revealed a large, previously unknown multiprotein complex called Myb-MuvB. The Myb DNA-binding protein is encoded by the sole Drosophila homologue of the vertebrate Myb oncogene family which is absent in the nematode C elegans. The other proteins in the complex are encoded by Drosophila homologues of the synMuvB genes which were discovered in C elegans due to loss-of-funotion mutants that cause a multivulval phenotype identical to that caused by gain-of- function mutants in the RTK-RAS-MAP kinase pathway. Both the Rb and RAS pathways contain critical targets for mutation in the development of most human cancers. In addition, recent screens for molecular markers in human breast cancer have revealed that increased expression of B-MYB/MYBL2 (the human orthologue of Drosophila Myb) is a strong predictor of poor prognosis. Therefore, better understanding of the Myb-MuvB complex will likely lead to improvements in the diagnosis and treatment of human cancer. We propose to use genetic, cell biological, and biochemical analyses in Drosophila to determine the consequences of acute loss or gain of function of components of the Myb-MuvB complex. Our preliminary data show that a failure of the normal progression of chromosome condensation occurs in the absence of Myb. In addition, we found that the deposition of the centromere-specific histone H3 variant CID/CENP-A is altered in the absence of Myb. Our hypothesis is that other components of the Myb-MuvB complex are also required for these critical steps in chromosome biology. Therefore, our specific aims are: (1) To determine which components of Myb-MuvB regulate chromosome condensation;(2) To delineate the role of Myb-MuvB in the establishment, maintenance, and spread of heterochromatin;(3) To test whether components of Myb- MuvB regulate the exchange of histone H3 variants.