Nucleotide biases of the mutational mechanism. Normalized mutation frequencies were calculated by comparing the fraction of mutations in a given nucleotide to the fraction of that nucleotide in the germ line sequences. If mutations occurred in a nucleotide at the same relative rate that it appeared in the germ line V_H sequences, it would have a normalized mutation rate of 1. When all mutations are considered, there is a slight bias away from mutating T nucleotides. However, when only the silent redundant positions are considered, there is a pronounced bias against mutations in C nucleotides in both B-CLL (top) and normal (bottom) sequences.