AbstractRAD21encodes a key component of the cohesin complex, and variants inRAD21have been associated with Cornelia de Lange Syndrome (CdLS). Limited information on phenotypes attributable toRAD21variants and genotype–phenotype relationships is currently published. We gathered a series of 49 individuals from 33 families withRAD21alterations [24 different intragenic sequence variants (2 recurrent), 7 unique microdeletions], including 24 hitherto unpublished cases. We evaluated consequences of 12 intragenic variants by protein modelling and molecular dynamic studies. Full clinical information was available for 29 individuals. Their phenotype is an attenuated CdLS phenotype compared to that caused by variants inNIPBLorSMC1Afor facial morphology, limb anomalies, and especially for cognition and behavior. In the 20 individuals with limited clinical information, additional phenotypes include Mungan syndrome (in patients with biallelic variants) and holoprosencephaly, with or without CdLS characteristics. We describe several additional cases with phenotypes including sclerocornea, in which involvement of theRAD21variant is uncertain. Variants were frequently familial, and genotype–phenotype analyses demonstrated striking interfamilial and intrafamilial variability. Careful phenotyping is essential in interpreting consequences ofRAD21variants, and protein modeling and dynamics can be helpful in determining pathogenicity. The current study should be helpful when counseling families with aRAD21variation.