We are very grateful to all the families who participated in this study, as well as their doctors, particularly Y. Dror, A. Durandy, T. Green, T. Gungor, J. Liu, I. Neri, and E. Plouvier, for referring samples to us. We thank the staff of the Genome Centre at Barts and The London School of Medicine and Dentistry and the Genomics Facility at King’s College London for help with DNA sequencing. We thank our colleagues D. van Heel, D. Kelsell, and V. Rakyan for help and advice. We thank K. Riha for providing protocols. This work was supported by the Wellcome Trust and the Medical Research Council.