Unilateral Ig H chain usage in aged IgH.TEμ heterozygous mice. (A) Flow cytrometric analysis of surface CD19, IgM^a, and IgM^b expression on peripheral blood mononuclear cells from 3 individual IgH.TEμ mice, T1, T2, and T3, representative for mice analyzed that developed unilateral Ig H chain usage over time. The CD19⁺ fraction was gated (shown only for mouse T1), and the IgM^a/IgM^b profile was analyzed (shown for all 3 mice). Data are shown as dot plots. Numbers indicate the percentages of cells within the specified populations. Age of the animal at the time of analysis is indicated by the numbers above the plots. (B) Total white blood cell count in IgH.TEμ mice showing random usage (RAN, n = 17) of IgM^a and IgM^b alleles, or unilateral usage of IgM^b (UNI, with < 10% IgM^a and > 90% IgM^b B cells, n = 18; P < .001, Mann-Whitney U test). The RAN mouse group consists of IgH.TEμ (< 5 months of age) as well as IgH.T mice (< 9 months). The age of onset of UNI group is indicated in Table S1. Each symbol indicates an individual mouse (■, RAN; and ▴, UNI). Bars show average with SD of 17 (RAN) and 18 (UNI) animals. (C) Giemsa staining of blood smear of a UNI mouse, showing the presence of large cells with little chromatin. Size marker represents 20 μm.