Cytosine-to-thymine transition substitutions are increased in X-HIgM B cells. The number (first row) and percentage (second row) of individual nucleotide substitutions detected in the variable segment of (A) nonproductive and (B) productive VDJH rearrangements in PB B cells from 4 NDs and 7 patients with X-HIgM are presented. The tables representing the percentages of all mutations scored are corrected for base composition of the target sequence. *Significant difference in C > T transitions; †significant difference in G versus C mutation frequency; ‡significant difference in A versus T mutation frequency.