-Alkyl-polyethylenimine 2 kDa (PEI2k)–stabilized superparamagnetic iron oxide (SPIO) nanoparticles, abbreviated as alkyl-PEI2k/SPIO, is a contrast agent that was synthesized by Liu et al. for stem cell labeling and tracking with magnetic resonance imaging (MRI) (1). MRI of SPIO-labeled stem cells has been considered to be the best choice for cell tracking because of its ability to follow the labeled cells for months as well as to provide detailed anatomic information with high resolution and soft tissue contrast (2, 3). Cells are labeled by direct injection of the SPIO-based agents into the tissue area of interest; cells are labeled by intravenous administration of the agents (4, 5). In both preclinical and clinical situations, cells are typically labeled for cell-tracking studies (4). Generally, SPIO nanoparticles need to be coated with organic polymers or other materials for efficient cell labeling. This surface coating increases the stability of SPIO nanoparticles and allows further chemical modifications with ligands (1, 4). In addition, the surface coating could influence the SPIO distribution within cells, which would thus influence the T2 relaxivity. However, there are still a lot of challenges to turn MRI cell tracking into a robust technique either in preclinical settings or in clinical applications. For example, it is impossible to discriminate live cells from dead cells, and to differentiate transplanted cells from macrophages. MRI quantification of both iron oxide concentration and cell number is also not reliable. It has been shown that SPIO particles clustered in dense vacuoles yield better local contrast enhancement than SPIO particles distributed in cytoplasm (1, 6). Controlled clustered SPIO nanoparticles can greatly shorten T2 relaxation time in comparison with single SPIO nanoparticles at the same iron concentration (1, 6). -Alkyl-polyethylenimine 25 kDa (PEI25k) is a commercial reagent used for gene transfection. Wang et al. applied PEI25k to stabilize SPIO, and the generated nanoparticles exhibited a controlled clustering structure, efficient cell uptake, and high T2 relaxivity when they were used to label mesenchymal stem cells (MSCs) (6). However, PEI25k has been found to be toxic to labeled cells, resulting in cell death, apoptosis, and differentiation inhibition. In comparison, lower molecular weight PEI, such as PEI2k, is more biocompatible. Liu et al. applied PEI2k to form stable nanocomplexes with SPIO (1). The generated nanoparticles, alkyl-PEI2k/SPIO, could hold multiple SPIO particles with a controlled clustering structure. Labeled MSCs showed no evident toxic effect on their viability, proliferation, and differentiation capacity.