Feng, SiqianPolycomb group (PcG) proteins are conserved epigenetic regulators that maintain targets at a repressed transcription state. In my dissertation research, I generated <italic>ph<super>del</super></italic>, the first real null allele of the <italic>Drosophila</italic> PcG gene <italic>polyhomeotic (ph)</italic>. Using this allele, I found that loss of <italic>ph</italic> causes cell over-proliferation in mosaic tissues in a non-autonomous manner, meaning that the mutant cells induce over-proliferation of neighboring wild type cells. I further identified the underlying signaling pathways: Notch signaling is elevated autonomously in <italic>ph<super>del</super></italic> cells, which causes the over-production of 3 Upd homologs. These ligands are then secreted and activate the JAK/STAT pathway in neighboring cells, which eventually causes cell over-proliferation. In addition, <italic>ph<super>del</super></italic> cells maintain normal cell polarity but undergo invagination to form unique 3 dimensional structures. Such structures are morphologically and functionally similar to epithelia-derived endocrine glands. Interestingly, <italic>ph<super>505</super></italic>, another <italic>ph</italic> allele that has long been considered null, causes both autonomous and non-autonomous cell proliferation in mosaic tissues. To explain the discrepancies between <italic>ph<super>del</super></italic> and <italic>ph<super>505</super></italic>, I characterized the nature of <italic>ph<super>505</super></italic>. Data from embryonic lethal stage, rescue by <italic>ph-d</italic>, and exon sequencing all showed that <italic>ph<super>505</super></italic> is a hypomorph. Functional analysis then proved that the same signaling pathway also underlies non-autonomous proliferation in <italic>ph<super>505</super></italic> mosaic tissues. I then showed that <italic>ph<super>505</super></italic> cells still respond to the Upd ligands they secreted, but <italic>ph<super>del</super></italic> cell are no longer responsive. This explains why <italic>ph<super>505</super></italic> cells still over-proliferate but <italic>ph<super>del</super></italic> cell do not. Next, Real-Time PCR results demonstrated that the JAK/STAT pathway receptor <italic>domeless</italic> has a higher expression level in <italic>ph<super>505</super></italic> cells than in <italic>ph<super>del</super></italic> cells, which may explain their different sensitivities to Upd ligands. Finally, genome wide ChIP data in public database suggest that <italic>Notch</italic> may be a direct target of Polycomb Repressive Complex 1, in which Ph is a core component. My dissertation established that loss of <italic>ph</italic> causes non-autonomous over-proliferation, and elucidated the underlying mechanism. My results also call for a reevaluation of the non-autonomous over-proliferation pathway in <italic>Drosophila</italic>. Finally, the fact that different alleles of the same gene cause tumors in very different ways have certain implications to cancer research and treatment.DissertationGeneticsCellular biologyMolecular biologycell proliferationJAK/STATNotchPolycomb group genespolyhomeotictumor