Kenneth M Yamada1, Roberto Mayor2. 1. Laboratory of Cell and Developmental Biology, Cell Biology Section, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892, USA. Electronic address: kyamada@dir.nidcr.nih.gov. 2. Department of Cell and Developmental Biology, University College London, Gower Street, London WC1E 6BT, UK. Electronic address: r.mayor@ucl.ac.uk.