Cornelia de Lange syndrome (CdLS) is genetically heterogeneous and is usually sporadic, occurring approximately once per 10,000 births. CdLS individuals display diverse and variable deficits in growth, mental development, limbs, and organs. In the past few years it has been shown that CdLS is caused by gene mutations affecting proteins involved in sister chromatid cohesion. Studies in model organisms, and more recently in human cells, have revealed, somewhat unexpectedly, that the developmental deficits in CdLS likely arise from changes in gene expression. The mechanisms by which cohesion factors regulate gene expression remain to be elucidated, but current data suggest that they likely regulate transcription in multiple ways.