The condensin complex functions both during mitosis, to form condensed mitotic chromosomes, and during interphase, to regulate expression of genes. Both of these goals are thought to be accomplished by altering the higher order of structure of chromosomes. In the nematode, C. elegans, two distinct condensin complexes perform these two functions. The mitotic condensin complex plays a role in chromosome segregation, while the dosage compensation complex regulates expression of genes on hermaphrodite X chromosomes to equalize X-linked gene dosage between the sexes. Our long-term goal is to decipher the molecular mechanism of worm dosage compensation and to determine how it relates to the mechanism of chromosome condensation. Biochemical purification of the dosage compensation complex led to the discovery of CPG-1, a new dosage compensation complex subunit, which also functions in chromosome segregation. To reveal mechanistic similarities between dosage compensation and chromosome segregation, we will use genetic, biochemical, and microscopic tools to analyze the ways in which CPG-1 can function during both processes. The second part of our proposed study will analyze the chromatin fiber of dosage compensated chromosomes. We found that HTZ-1, a histone variant of H2A, is depleted on dosage compensated X chromosomes as compared to autosomes. The goal of our proposed experiments is to decipher how this depletion contributes to dosage compensation. To do that we will study the relationship between the association of dosage compensation complex and HTZ-1 with the X chromosome, both at low resolution (immunofluorescence) and high resolution (chromatin immunoprecipitation). Healthy development of any organism depends on delivering the correct number of chromosomes to each cell and on each cell then turning on the right set of genes. Delivering the wrong number of chromosomes to cells, or inappropriately turning genes on or off, leads to developmental disorders or cancer. These functions of chromosomes depend largely on the structure and organization of DMA contained in them. A better knowledge of the basic biological principals of how chromosome structure determines gene activity levels and ensures proper chromosome segregation will be important to understanding both healthy development and the development of disease.