Mutation frequency of <i>Jh4</i> downstream intron of <i>Igh</i> of B220⁺ CD3⁻ cells.

P value (two tail Fisher's exact test): a, p = 3.2×10−8; b, p = 2.3×10−22. Statistical comparisons between two types of cells (a- and b-pairs) were done by use of 2×2 contingency tables made by Mutated base/Total base of each pair.