Cover photograph (Copyright © 2008, American Society for Microbiology. All Rights Reserved.): Activated (phosphorylated) extracellular signal-regulated kinase is detected by immunochemistry in cartilage almost exclusively in quiescent mature hypertrophic chondrocytes (black signal). The suppression of this activation by either parathyroid hormone-related peptide or a MEK1/2 inhibitor is associated with a delay in hypertrophic differentiation, whereas cell proliferation is not affected. (See related article in January 2008, vol. 28: issue 1, p. 344.)