Biochemical and genetic studies have demonstrated that transcription factors encoded by the E2A gene are essential in regulating B lineage specific gene expression and B lineage commitment. However, the mechanism by which E2A regulates B lineage commitment is not known. It has been reported that E2A controls B lineage commitment in a dosage dependent manner. To further investigate this gene dosage effect, we analyzed E2A expression during normal B cell development in mice carrying a functional E2AGFP knockin allele. Mice carrying this fusion allele were examined for E2A gene expression during bone marrow B cell development. A dramatic upregulation of E2A is observed concomitant with the initiation of immunoglobulin heavy chain D-J rearrangement and the induction of Early B cell Factor (EBF) gene expression. We also show that this E2A upregulation does not occur in the absence of the EBF gene. These results indicate that E2A upregulation is a critical step in regulating B-lineage commitment. It further suggests that E2A gene dosage may be determined by a cross regulation between E2A and EBF during B lineage commitment.