Lamin B1 plays an important role in the nuclear envelope stability, the 
regulation of gene expression, and neural development. Duplication of LMNB1, or 
missense mutations increasing LMNB1 expression, are associated with 
autosomal-dominant leukodystrophy. On the basis of its role in neurogenesis, it 
has been postulated that LMNB1 variants could cause microcephaly. Here, we 
confirm this hypothesis with the identification of de novo mutations in LMNB1 in 
seven individuals with pronounced primary microcephaly (ranging from -3.6 to -12 
SD) associated with relative short stature and variable degree of intellectual 
disability and neurological features as the core symptoms. Simplified gyral 
pattern of the cortex and abnormal corpus callosum were noted on MRI of three 
individuals, and these individuals also presented with a more severe phenotype. 
Functional analysis of the three missense mutations showed impaired formation of 
the LMNB1 nuclear lamina. The two variants located within the head group of 
LMNB1 result in a decrease in the nuclear localization of the protein and an 
increase in misshapen nuclei. We further demonstrate that another mutation, 
located in the coil region, leads to increased frequency of condensed nuclei and 
lower steady-state levels of lamin B1 in proband lymphoblasts. Our findings 
collectively indicate that de novo mutations in LMNB1 result in a dominant and 
damaging effect on nuclear envelope formation that correlates with microcephaly 
in humans. This adds LMNB1 to the growing list of genes implicated in severe 
autosomal-dominant microcephaly and broadens the phenotypic spectrum of the 
laminopathies.