Abstract
Recombinant DNA constructed from unit length BK virus DNA and from several defective viral forms was cloned in Escherichia coli HB101. The cloned unit length BK virus DNA retained its infectivity for human embryonic kidney cells, whereas the cloned defective DNA showed no infectivity. Restriction endonuclease digestion of cloned defective DNA and purified virus DNA indicated that many of the defective forms contain re-iterated sequences.