The 101 occurrences of the tripeptides Asn-X-Ser and Asn-X-Thr in the available protein sequence data are tabulated; carbohydrate is found, attached to the asparagine, in not more than 20 of the 101 tripeptides. A statistical analysis of the data from all completely sequenced proteins shows that the observed frequency of occurrence of the two kinds of tripeptides is only about 65% of the expected. This lowered frequency is evidence for a newly postulated kind of limitation-which we call a "restricted sequence"-imposed by natural selection on the primary structure of proteins. © 1970.
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