Prominent among these TFs are the ets-family TF PU.1, Ikaros, zinc finger TF Bcl11a, helix-loop-helix TF E2A, EBF1 (early B cell factor 1), and paired box TF Pax5.8,9 Both E2A and EBF1 have been reported to regulate B lineage specification and commitment, and Pax5 activity is vital for B lineage maintenance.10–15 Genetic ablation of these TFs has revealed serious abnormalities in B cell development, leading to severe B cell lymphopenia.9 Here, PAX5 is linked to Decreased total B cell count.