The defect in lymphocyte development in Mysm1^tm1a/tm1a mice is because of a cell-intrinsic function of Mysm1 in hematopoietic cells. (A-B) Mysm1-promoter activity in (A) thymocytes, and (B) B-lineage BM cells. The measurements done on phenotypically normal Mysm1^+/tm1a cells that carry a single copy of the Mysm1 promoter–driven β-galactosidase reporter. The data are presented as fold difference in β-galactosidase activity in Mysm1^+/tm1a versus background activity in wild-type cells; bars show means ± SEM from ≥ 3 mice. β-galactosidase activity was significantly above background in Mysm1^tm1a/+ double-negative DN1-DN4 and double-positive thymocytes, as well as in B-lineage cells including B220⁺IgM⁻IgD⁻ pre/pro-B cells, B220⁺IgM⁺IgD⁻ immature and B220⁺IgM^lowIgD⁺ mature B cells (P < .05, t tests), although β-galactosidase activity in the single-positive CD4 and CD8 thymocytes was not significant. (C-F) Irradiated Rag1^−/− mice were reconstituted with a 1:1 mix of CD45.1⁺ wild-type and CD45.2⁺Mysm1^tm1a/tm1a BM for 8 weeks; data were from 4 animals per group and were reproducible in 2 independent experiments. (C) Numbers of thymocytes, and (D) BM B cells, including pro/pre-B220⁺IgM⁻IgD⁻, immature B220⁺IgM⁺IgD⁻, and mature B220⁺IgM^lowIgD⁺ B cells, in the chimeric mice. Bars represent means ± SEM; ANOVA analyses indicated significantly increased contribution of wild-type versus Mysm1^tm1a/tm1a cells to the CD4⁺CD8⁺ double-positive (P < .001) and the single-positive CD4 (P < .01) and CD8 (P < .05) populations in the thymus, as well as to the pre/pro-, immature, and mature B cells in the BM (P < .01 for all). (E-F) Representative flow cytometric profiles of the spleen of the chimeric mice, gated on CD45.1⁺ wild-type cells (left) or CD45.1⁻Mysm1^tm1a/tm1a cells (right), and stained (E) for CD4 and CD8, or (F) for B220, IgM, and IgD.