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In naturally occurring strains and laboratory selected variants of echovirus type 9, the association between virulence for infant mice (v) and the reproductive capacity at 40° (t) was of the v+t+ and v−t− types. By different selection methods the two other marker combinations, v+t− and v−t+, were obtained. The frequency of t+ in a t− echovirus type 9 population was 10−5. Of the four possible combinations between the v and c (cytopathic effect for human epithelial cell lines) markers, only three were found. The v+c+ covariation could not be obtained in spite of appropriate methods of selection.
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