SMC1A and Cornelia de Lange syndrome: This has been attributed to the fact that the SMC1A/SMC3 mutations have a predicted modest affect on protein structure—more severe mutations are assumed to be incompatible with birth—but it could also reflect a fundamental difference in the cellular functions of NIPBL and cohesin and a different aetiology for NIPBL-associated CdLS and the CdLS-like diseases associated with mutations in cohesin components.