In human, ATR gene is essential in development and its deficiency resulted in a rare autosomal recessive disorder called Seckel syndrome (Table 3), which is featured by intrauterine growth retardation, microcephaly, and developmental defects.150 ATR kinase belongs to the phosphatidylinositol 3-kinase-related kinase (PIKK) family and functions as the apical responder to ssDNA exposure.151 ssDNAs are abundant in numerous physiological processes including DNA replication, HR, NER, and cancer cells with replication stress. This evidence concerns the gene ATR and cancer.