Using data from DNA films at Γ = 2.5, the modification factors M(X), with X = Ade, Thy, and Cyt, are plotted against the frequency of finding no Gua within 2 bases of X. Table S1 of Supporting Information gives the frequencies for all possible base quintets, with the sum of the 1024 possibilities equal to 1. This was done for the forward and reverse strands in C. perfringens, bovine (calf thymus) DNA, pUC18, and Micrococcus sp. 28 plasmid pSD10 (68.0% GC). The latter was the closest we could find to M. luteus (72% GC), the entire genome of which had not been sequenced. The fbr data for pUC18 was reported previously.30 For M(Cyt), previous data 14 on the 20-mer [d(CG)10]2 was used for the zero frequency point.