Immune System Thymus (image) Max-Planck-Gesellschaft Caption The normal mouse thymus (left) contains only a small fraction of B-cells (red). If the gene FOXN4 is activated, a fish-like thymus with many B-cells develops. This state is likely to have existed about 500 million years ago, at the time when the first vertebrates emerged. Credit Max Planck Institute of Immunobiology & Epigenetics Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.