Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Artistic representation of a chromosome with telomere loops and extrachromosomal telomeric repeat DNA circles (t circles) from a human alternative lengthening of telomeres (ALT) cell. Electron micrographs of t circles isolated from ALT cells are on the right. (Image prepared by Marian Miller, University of Cincinnati.) (See related article on p. 9948.)