BIRC5 and neural tube defect: The deletion of the Survivin gene specifically in Flk-1+ (vascular endothelial growth factor receptor 2 (VEGFR2) or Flk-1, hereafter referred to as Flk-1) progenitors, which first appear in the yolk sac mesoderm on mouse embryonic day 7 (E7)6, during embryonic organogenesis results in NTD formation7, suggesting that survivin produced by the Flk-1+ progenitors can be packaged in exosomes and thus acts on neuroepithelial cells in promoting neurulation.