In patient-isolated fibroblasts, reduced phosphatase activity of PNKP mutants and inability to repair 3′-PO single-stranded DNA breaks correlated with neurodevelopmental disorders (MCSZ), whereas reduced kinase activity in the context of functional phosphatase correlated with milder neurodegenerative pathologies (44). The gene discussed is PNKP; the disease is neurodevelopmental disorder.