We thank Rudi Balling (GBF Research Center) and Shoumo Bhattacharya (University of Oxford) for many helpful and stimulating discussions. We thank Evi Wollscheid-Lengeling (GBF) for help with harvest of neonatal mice, Ronald Frank (GBF) for providing Ptdsr peptide arrays, Maria Ebel (GBF) for ES cell blastocyst injections, Manfred Rohde (GBF) for electron microscopy, Kurt Dittmar (GBF) for help with confocal microscopy and Bastian Pasche (GBF) for critical reading of the manuscript. We thank BayGenomics, a genomic consortium funded by the US National Heart, Lung, and Blood Institute, for providing the ES cell gene-trap line RRJ099. This work was supported in part by the EU project EUMORPHIA, "Understanding human molecular physiology and pathology through integrated functional genomics in the mouse model" (QLG2-CT-2002-00930).