Germ cells give rise to all the cells of subsequent generations and to accomplish that must differ from somatic cells in several different respects. Germ cells are totipotent, with stem-cell-like characteristics, and can be considered to be immortal since they generate the germ cells of future generations. Immortality is also characteristic of stem cells and most transformed or cancerous cell types. Research into the molecules that confer upon germ cells their unique traits will provide insight into the genetic control mechanisms that keep cancerous cells undifferentiated and able to divide indefinitely. Large granules composed of RNA and protein, referred to as P granules in the nematode Caenorhabditis elegans, are a consistent and unique feature of germ cell cytoplasm in many different organisms. New P-granule components will be identified using a forward genetic screen for mutations that impair P-granule assembly and by a reverse genetic candidate-gene approach. The genes identified will be molecularly analyzed and positioned in the P-granule assembly pathway; those that are the furthest upstream will be used to assess the basic functions of P granules during germline development. Fluorescence recovery after photobleaching (FRAP) experiments will be used to assess whether P granules are dynamic or relatively stable cellular structures in vivo.