CTC1 and cancer: Our data and those from a previous study in which knockout of CTC1 resulted in similar telomeric and proliferative defects in mouse embryonic fibroblasts (Gu et al., 2012) may seem at odds with studies demonstrating that knockdown of either Stn1 or Ctc1 did not result in telomere dysfunction or any significant growth defects in human cancer cell lines such as HeLa (Casteel et al., 2008; Miyake et al., 2009; Surovtseva et al., 2009) and HT1080 (Chen et al., 2012).