Cover photograph (Copyright © 2007, American Society for Microbiology. All Rights Reserved.): Mammary tumor cell lines derived from engineered Bloom\'s syndrome mice exhibit chromosomal instability. The blue DAPI (4′,6′-diamidino-2-phenylindole)-stained nucleus was obtained from a tumor line (1578-2) exhibiting profound chromosome missegregation defects. It is multilobed and multinucleated and contains two or three chromatin bridges. Superimposed on this are chromosomes from a tumor culture (D-11) that was assayed for time-dependent acquisition of chromosomal instability. The Giemsa-stained spread, obtained from late-passage cells, is full of chromosomal structural abnormalities seen in magenta as 14 metacentrics and two small marker chromosomes. Normal chromosomes either are shown in green false color or are hidden. (See related article in September 2006, vol. 26: issue 17, p. 6713.)