Cover image: Intravital microscopy image of fluorescent aptamer sensor-modified mesenchymal stem cells that homed to bone marrow niche. These in vivo sensors can potentially be used to visualize cellular functions and interactions in real time in living animals (work from laboratories of Dr Weian Zhao (University of California, Irvine, CA), Dr Jeffrey Karp (Brigham and Women’s Hospital, Boston, MA) and Dr Charles Lin (Massachusetts General Hospital, Boston, MA)).