The expression and function of histone deacetylases (HDAC) are well known during embryonic development and especially plays a pivotal role in the development of the nervous system. HDAC inhibition during embryonic development has been correlated to several congenital malformations mainly affecting neurodevelopment. However, the kind of malformation strongly depends on the timing of disturbance, i.e. when during embryonic development the exposure occurred. This AOP concentrates on disturbances by HDAC inhibition during the first weeks of neurodevelopment, before or around the time point of neural tube closure. Therefore, this AOP suggests a mechanism how HDAC inhibitors could lead to the observed neural tube defects. It assumes that HDAC inhibition leads to an imbalance of histone modifications and eventually to altered gene expression. In the next KE altered gene expression may lead to a wrong differentiation of neuroectodermal cells that cannot close the neural tube anymore and therefore leads to neural tube closure defects. This AOP is linked to case study 2 that investigates the effects of VPA and its structural analogs the EU-ToxRisk DART (development and reproductive toxicology) test methods.