Germ line mutations of two critical DNA damage response (DDR)-orchestrating phosphatidylinositol kinase-related kinase (PIKK) family kinases, ATM and ATR, cause progressive cerebellar dysfunction in ataxia-telangiectasia (47) or microcephaly with dwarfism in Seckel syndrome type I, respectively (48) in, addition to DNA damage hypersensitivity. Here, ATR is linked to microcephaly.