Immunologists who study lymphocyte development tend to identify themselves with either the “T” or “B” lymphocyte. In recent years, however, it has become clear that the transcription factors that control lineage specification are much more promiscuous than those who study them. Researchers working on transcription factors in both T and B lymphocyte development gathered together for what is probably the first time to discuss recent developments in the field (Madrid, October 19–21, 1998). The meeting was organized by JM Redondo (Centro de Biologia Molecular “Severo Ochoa,” Madrid), S. Pettersson (Karolinska Institute, Stockholm) and P. Matthias (Friedrich Miescher Institute, Basel) and held at the Juan March Foundation in Madrid, Spain.