On the Cover: SK-BR-3 cells transduced with a recombinant retrovirus encoding a kinase-defective mutant form of the Chk2 checkpoint kinase fused to GFP (GFP-Chk2^KD). Overlay of fluorescence and bright field microscopy images shows nuclear localization of the fushion protein in proliferating cells. For details, see the article by Aliouat-Denis, et al. on page 627.