Accumulation of monoclonal B-cell populations in aged IgH.T and IgH.TEμ mice. (A) Spleen size of WT (▾, n = 6), RAN (▴, n = 18), and UNI (■, n = 33) animals. Each symbol indicates an individual mouse; mean values are indicated by a line. * Significant difference with WT (P < .001; Mann-Whitney U test). (Ages of mice are given in Table S1). (B) Flow cytometric analysis of total spleen cells in the 3 UNI mice indicated. Numbers indicate the percentage of CD19⁺ B cells. (C) Expression of Igκ or Igλ L chain in splenic IgM⁺ B cells of 3 UNI mice. Numbers indicate the percentage of cells within the specified quadrants. (D) Schematic representation of WT and targeted alleles, and position of restriction sites used for Southern analysis of Ig H chain recombination. Horizontal line represents the position of the XX1.2 probe, spanning the DQ-J_H region. S indicates SacI; X, XbaI. (E) DNA rearrangements in total spleen cells of UNI animals. DNA was digested with XbaI or SacI and hybridized to the XX1.2 probe. Tail DNA from WT and UNI animals was used as reference for the WT germline fragment (GL) and the knockin germline fragment (KI). Note that digestion with XbaI results in 2 germline KI fragments. A vertical line has been inserted between samples T4 and T5 (SacI digests) to indicate a repositioned gel lane.