DNA2 and microcephalic primordial dwarfism: Like mitochondrial myopathy, Seckel syndrome is genetically heterogeneous and disease mutations affect groups of genes with roles in DNA replication/repair (ATR, DNA2, CTIP, NSMCE2, and TRAIP) and segregation of the replicated genome into daughter cells (CEP63, CEP152, CENPJ, NIN, and NSMCE2).