Moreover, like Sld2, chromatin association by DONSON requires double hexamer formation, the kinase activity of DDK and CDK as well as the presence of TopBP1/Dpb11.41,42 Mutations in DONSON are the cause of microcephalic primordial dwarfism, a group of disorders characterized by growth delays, often related to defects in the initiation step of DNA replication.43,44 A role for DONSON in origin activation is remarkable as it indicates a function separate from its established role during elongation. The gene discussed is TOPBP1; the disease is microcephalic primordial dwarfism.