In addition, PCNA mRNA levels were only moderately increased (1.4-fold) in lung cancer compared to those of Claspin and CHK1 (4.5- and 4.4-fold, respectively; Fig. 1b), indicating that the increased expression of Claspin, Timeless, and CHK1 in cancer cells does not simply reflect increased proliferation. Here, CHEK1 is linked to cancer.