Although magnetic nanoparticles are being used more and more in cell imaging and tissue bioengineering, what happens to them within stem cells in the long term remained undocumented. Researchers from CNRS, the Sorbonne Université, and universities Paris Diderot and Paris 13, have shown substantial degradation of these nanoparticles, followed in certain cases by the cells “re-magnetizing.” This phenomenon is the sign of biosynthesis of new magnetic nanoparticles from iron released in the intracellular medium by the degradation of the first nanoparticles. Published in PNAS on February 11, 2019, this work may explain the presence of “natural” magnetism in human cells, and help to envisage new tools for nanomedicine, thanks to this magnetism produced by the cells themselves.