ATM and ATR orchestrate overlapping DNA damage responses in reply to different forms of DNA strand discontinuities. But, knockout mouse models suggest that ATR is essential for viability in contrast to ATM. Recently, more sophisticated mouse models have been published including a conditional ATR-knockdown system and by modelling the human ATR-Seckel syndrome-causative mutation. Here, I will overview and contrast these models highlighting the advances both represent in our understanding of how defects in the ATR-dependent DNA damage response can impact on normal development, tissue homeostasis, ageing and cancer.