Holoprosencephaly (HPE) is the most common forebrain defect in humans characterized by failure of the cerebral hemispheres to completely separate into two distinct halves. The wide HPE spectrum encompasses distinct brain malformation types (from alobar HPE to microform), various craniofacial defects (from cyclopia to normal face) and other extra-craniofacial defects. In addition to this clinical variability, the molecular basis underlying HPE are very heterogeneous (more than 10 genes involved in HPE). Here, we report the correlations between the clinical findings and the molecular analyses of the largest European series (645 HPE patients) and propose a new algorithm for gene analysis and genetic counselling in this complex disease. (By Sandra MERCIER, http://jmg.bmj.com/content/early/2011/09/22/jmedgenet-2011-100339.abstract?papetoc )