Szybalski's rule says that lower-protein particles like viruses contain more purines than pyrimidine in their nucleic acid sequence.
[1] This is to prevent double-stranded RNA formation of one or two separate RNA strand that have complementary regions.
The formation of a double-stranded RNA is not efficient for viruses as it may delay or stop RNA replication or protein formation.
[2] The rule is named for Wacław Szybalski.
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