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In virus-infected or transformed cells, polyoma expresses a large number of functions. Attempts to understand these in relat ion to the structure of the virus DNA have unti l recently been l imited by our knowledge of polyoma DNA anatomy and nucleo-tide sequence. Methods such as denaturat ion mapping, based on the variat ion of base compo-sition in different regions of the DNA, provide some information, but they are l imited by the small size and circular i ty of the molecule (Follet and Crawford 1968). Cleavage by restrict ion endonucleases, which cut the DNA at specific nucleotide sequences, provide both better reference points on the DNA and the means for physical separation of different regions. Here we have made use of a variety of
Crawford et al. (Tue,) studied this question.