Strains of Neurospora having chromosome rearrangements of various types are being studied genetically and cytologically, and applied to a variety of problems. Several types (e.g., insertional translocations) recurrently generate via meiosis a class of viable progeny containing nontandem duplications; their properties are of interest for comparison with similar rearrangements in diploid organisms, including man, where direct study is difficult. Duplication-generating aberrations are being used for studying vegetative incompatibility, regulation, complementation and dominance, and mapping by duplication coverage. Many duplications show instability due to chromosome breakage, which is subject to genetic modification. These are being used in a search for control systems that regulate breakage. Acentric chromosome fragments which can replicate are produced by crossing over in inverted transpositions. Their behavior and possible genetic role are being examined. Probable diploids have been obtained, but they are unstable; a suppressor of vegetative incompatibility will be used in attempting to obtain stable diploids. Additional strains from nature will be used as a source of new genic and chromosomal variants. A search for abnormal nucleases will employ strains showing defective repair mechanisms or high chromosome breakage.