Our laboratory wishes to define, localize and study the mode of action of genes involved in susceptibility/resistance to x-irradiation-induced leukemia. We have recently identified the major locus involved in susceptibility to fractionated irradiation (FX)-induced leukemia, Ril-1. We have now detected polymorphism among congenic mouse strains differing at Ril-1 with a cloned DNA probe specific for the spleen focus forming virus (SFFV). When DNA from these congenic mice is digested with restriction enzyme PvuII, a 5.7 kb fragment is present is susceptible (C57BL/6) but not resistant (B6.C-H-30c) mice. Since these mice are identical at most genes, except Ril-1, and tightly linked genes, the PvuII fragment in question must be encoded by Ril-1 or tightly linked loci. Further studies have shown that the probable distance of the fragment from Ril-1 is 0-60 kilobases. Therefore, it now appears possible to clone Ril-1. Starting with the 5.7 kb PvuII fragments we propose to first derive a single copy probe specific for cellular sequences in the Ril-1 region, then isolate the complete gene from lambda and/or cosmid libraries. Such gene(s) will be micro-injected into embryos, and transfected into transplanted bone marrow cells in order to unequivocally demonstrate their functional role in FX-induced leukemia. In addition, experiments will be performed to try to understand the mechanism of Ril-1 action in FX-induced leukemia. These experiments will include measurement of (1) temporal changes in mRNA levels or size of transcripts, (2) rearrangements or translocation of Ril-1 DNA sequences, (3) the activation of Ril-1 on subsets of lymphocytes, and (4) the development of serological reagents for Ril-1 to aid in biochemical and cellular studies of the gene product(s).