By tracking cells' movements in whole organisms, scientists hope to learn how cellular migration contributes to processes such as immune response, angiogenesis, and embryogenesis. Magnetic resonance imaging (MRI) provides a way of doing this non-invasively and with cellular resolution. However, current methods require attaching magnetic beads to the cells' surface, which leads to rapid clearance of the labeled cell. Lewin and colleagues have overcome this obstacle, using the Tat peptide of HIV, which has been shown to mediate efficient internalization of proteins into cells. Reasoning that it might also mediate internalization of iron particles used as magnetic labels, they attached the peptide to a triple magnetic, fluorescent, and isotopic label, and showed that the label was efficiently taken up by cells including CD34+ and neural progenitor cells. After injection into mice, the magnetic CD34+ cells homed to bone marrow, and once the bone marrow was harvested, the cells could be detected by MRI at single cell resolution, and then recaptured with magentic separation columns. (See p. 282 and 410.)