Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): Multilayering of MDCK cells elicited by active Raf is cell autonomous. MDCK cells expressing conditionally active enhanced green fluorescent protein-tagged Raf-1 (green) were mixed with parental MDCK cells (1:9 ratio) on transwell filters. Following Raf-1 activation (42 h), the cells were fixed and labeled with fluorescent phalloidin (red) to outline cell borders. The cells were then imaged by confocal microscopy in the x-y and x-z axes (upper and lower panels, respectively). When cultured under these conditions, only cells expressing active Raf-1 exhibited cell multilayering and signs of morphological transformation of polarized epithelial cells. (See related article in December 2000, issue 24: vol. 20, p. 9364.)