YY1 Overexpression Negatively Impacts B Cells Figure 3. Cells expressing high levels of YY1 are enriched in the LT-HSC cell population. Chimeric mice were made as described in Figs. 1 and 2. (A) MigR1-FlagYY1 reconstituted mice show a relative enrichment in the LSK fraction. MigRI and MigR1-FlagYY1 reconstituted bone marrow cells were sorted into GFP2 and GFP+ populations and the percentage of LSK cells in each fraction are plotted. (B) YY1 expressing cells are enriched in the LT-HSC fraction but not in the ST-HSC, CMP, or GMP fractions. In (A) and (B), cell fractions marked with two asterisks show significant differences between MigRI and MigR1-FlagYY1 populations at P,0.01. doi:10.1371/journal.pone.0030656.g003 retroviral over-expression as bone marrow myeloid lineage cells developed normally (Fig. 2). In vitro studies confirmed the in vivo results showing selective disadvantage of high YY1 expression in B lineage cell lines, but not in myeloid cell line 32D. Thus, while YY1 over-expression leads to growth disadvantage in B lineage cells, there is no negative impact on myeloid cells. Our study is the first to show that YY1 over-expression negatively impacts B cell development. It is interesting that loss of YY1 [31] and overexpression of YY1 (our results here) both result in altered B cell development. This situation is somewhat reminiscent of the impact of overexpression verses knock-out of transcription factor GATA-3. GATA-3 is essential for T cell development and knock-out severely impacts hematopoiesis [36,37,38]. However, overexpression of GATA-3 also blocks lymphoid and T lineage development [39,40]. Thus, YY1 yields a similar scenario in which both loss of, and overexpression of YY1 impacts B cell development. However, the phenotypes of these two experimental systems are quite different. YY1 conditional knock-out results in arrested B cell development at the pro-B cell stage and reduced distal VH gene PLoS ONE | www.plosone.org 7 January 2012 | Volume 7 | Issue 1 | e30656