Methods for modifying specific regions of the polyoma virus genome are being developed. The effects of the modifications upon the physical and biological properties of both polyoma virus and SV-40 DNAs are being studied. Those treatments which prove to be mutagenic will be employed to isolate novel polyoma mutants which have not heretofore been isolated. Such mutants will be characterized by biochemical and genetic means to define viral functions important in replication and transformation. In addition, defective polyoma DNAs lacking substantial portions of the genome have been characterized. One type of covalently closed circular DNA contains a unique portion of the polyoma genome (including the origin of replication) tandemly repeated three to four times. Some of the biological and physical properties of this DNA have been defined.