E2A helix-loop-helix proteins are involved in the control of various developmental pathways. We show here by in situ hybridization that E2A transcripts are present in most embryonic and adult tissues. However, no E2A expression is detectable in heart and nonproliferative regions of the brain and spinal cord. Highest levels of E2A expression are found in the ependyma cell layer surrounding the cerebral ventricles in the embryonic rat brain. In addition, in the embryo, E2A transcripts were found in secretory cells of the pancreas, the bronchial tubes of the lung, glomeruli of the kidney, and the lining of the stomach. Interestingly, high levels of E2A transcripts are selectively found in the germinal center of the lymphatic nodules in the adult rat spleen. Thus, E2A, like its Drosophila homolog daughterless, is expressed in most tissues. The most notable feature of the E2A expression pattern is its high levels of expression in some areas of rapid cell proliferation and differentiation and in certain epithelial cell types.