Fig. 1. Cells in the neural folds of the mouse embryo undergo an epithelial-to-mesenchymal transition (EMT) to become migratory and give rise to the neural crest. According to Jim Weston's provocative new theory, cells delaminate from the neural epithelium and also from the non-neural ectoderm. The cell population that is proposed to originate from the non-neural ectoderm is shown in yellow and is characterised by the overlapping expression of platelet-derived growth factor (PDGFR, a mesenchymal cell marker) and low levels of cytoplasmic E-cadherin (a marker of non-neural epithelial cells), the latter being compatible with the non-neural origin of these cells. Its mesenchymal phenotype is also compatible with the presence of Snail, a zinc-finger transcription factor that is known to trigger the EMT (Nieto, 2002). Lineage analyses are necessary to confirm the origin of these cells and to assess the nature of the derivatives that they produce. dpc, days post coitum.