Several genetic disorders are characterized by normal head size at birth, 
followed by deceleration in head growth resulting in postnatal microcephaly. 
Among these are classic disorders such as Angelman syndrome and MECP2-related 
disorder (formerly Rett syndrome), as well as more recently described clinical 
entities associated with mutations in CASK, CDKL5, CREBBP, and EP300 
(Rubinstein-Taybi syndrome), FOXG1, SLC9A6 (Christianson syndrome), and TCF4 
(Pitt-Hopkins syndrome). These disorders can be identified clinically by 
phenotyping across multiple neurodevelopmental and neurobehavioral realms, and 
enough data are available to recognize these postnatal microcephaly disorders as 
separate diagnostic entities in their own right. A second diagnostic grouping, 
comprised of Warburg MICRO syndrome, Cockayne syndrome, and 
Cerebral-oculo-facial skeletal syndrome, share similar features of somatic 
growth failure, ophthalmologic, and dysmorphologic features. Many postnatal 
microcephaly syndromes are caused by mutations in genes important in the 
regulation of gene expression in the developing forebrain and hindbrain, 
although important synaptic structural genes also play a role. This is an 
emerging group of disorders with a fascinating combination of brain 
malformations, specific epilepsies, movement disorders, and other complex 
neurobehavioral abnormalities.