We will continue to use mutagens to induce paracentric and pericentric chromosomal inversions in different mouse strains and stocks. New inversions will be (1) characterized as to genetic location; (2) genetically marked, if possible, or placed with genetically marked homologous chromosomes; (3) studied with regard to cytological, physiological, and anatomical effects, and (4) combined, when possible, with other inversions or Robertsonian metacentric translocation, to produce test systems for precise estimates of mutational loads in irradiated populations. Specific proposed studies for this coming year are (1) to continue our attempts to induce pericentric inversions, (2) to use inversion heterozygotes to study the incidence of somatic recombination induced by certain mutagens, (3) to characterize more precisely the first lethal mutation induced by a mammalian inversion system, (4) to determine the genetic linkage of several new inversions, (5) to continue to characterize inversions cytologically using G-banding techniques, (6) to continue placing two inversions in a single stock in order to produce a more powerful mutagen testing system, and (7) to characterize inversions during meiosis for chiasma frequency and frequency of different types of inversion bivalents. BIBLIOGRAPHIC REFERENCES: Eicher, E. M., R. H. Stern, J. E. Womack, M. T. Davisson, T. H. Roderick, and S. C. Reynolds. 1976. Evolution of mammalian carbonic anhydrase loci by tandem duplication: close linkage of Car-1 and Car-2 to the centromere region of Chromosome 3 of the mouse. Biochem. Genet. (in press). Roderick, T. H. 1976. Inversions of mice in studies of mutagenesis. Genetics (in press) (abstract).