The molecular mechanisms that specify stem cell identity and regulate stem cell self-renewal and differentiation are crucial for realizing the vast potential of stem cells for medicine. We propose to identify molecular mechanisms that regulate stem cell behavior in the Drosophila male germ line as a model system. During the last funding period, we demonstrated that somatic support cells provide a crucial microenvironment that regulates both stem cell self renewal and differentiation. We will probe the possible stem cell intrinsic role of the upd receptor domeless, negative regulators of JAK/STAT pathway signaling, and the Zn-finger transcription factor escargot in regulating stem cell self-renewal. We will test whether the upd-JAK-STAT signaling pathway that specifies stem cell-self-renewal in the male germ line regulates stem cell behavior in other stem cell types. We will identify gene products expressed preferentially in stem cells by microarray analysis and test their possible function in stem cell self-renewal or differentiation by mutant and over-expression studies. We will test genes implicated in precursor cell self renewal vs. differentiation in blood for roles in regulating male germ line stem cell behavior, including the AML1 homolog lozenge (lz), and homologs of Bmi1, implicated in maintenance of mouse hematopoietic stem cells. Finally, we will phenotypically characterize newly identified mutants that affect stem cell specification, division, self renewal and differentiation and molecularly clone the most promising genes, placing their mode of action in the context of pathways already known to regulate male germ line stem cell behavior by double mutant and molecular epistasis experiments.