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Dormant human acute lymphoblastic leukemia (ALL) cells co-localize with extracellular matrix osteopontin (OPN) in the bone marrow of an NSG mouse (intravital confocal microscopy). Primary ALL cells were labeled with a fluorescent membrane dye (green) that is retained in dormant, noncycling cells but rapidly diluted to undetectable levels in proliferating cells. Extracellular OPN (red) was detected in vivo using fluorescent antibodies. Six weeks after ALL engraftment, the majority of dormant ALL cells were localized to regions of high OPN expression. See the article by Boyerinas et al on page 4821.