Cornelia de Lange syndrome (CdLS) is a multisystem genetic disorder with distinct facies, growth failure, intellectual disability, distal limb anomalies, gastrointestinal and neurological disease. Mutations inNIPBL, encoding a cohesin regulatory protein, account for >80% of cases with typical facies. Mutations in the core cohesin complex proteins, encoded by theSMC1A,SMC3andRAD21genes, together account for ∼5% of subjects, often with atypical CdLS features. Recently, we identified mutations in the X-linked geneHDAC8as the cause of a small number of CdLS cases. Here, we report a cohort of 38 individuals with an emerging spectrum of features caused byHDAC8mutations. For several individuals, the diagnosis of CdLS was not considered prior to genomic testing.Most mutations identified are missense andde novo. Many cases are heterozygous females, each with marked skewing of X-inactivation in peripheral blood DNA. We also identified eight hemizygous males who are more severely affected. The craniofacial appearance caused byHDAC8mutations overlaps that of typical CdLS but often displays delayed anterior fontanelle closure, ocular hypertelorism, hooding of the eyelids, a broader nose and dental anomalies, which may be useful discriminating features.HDAC8encodes the lysine deacetylase for the cohesin subunit SMC3 and analysis of the functional consequences of the missense mutations indicates that all cause a loss of enzymatic function. These data demonstrate that loss-of-function mutations inHDAC8cause a range of overlapping human developmental phenotypes, including a phenotypically distinct subgroup of CdLS.