Monarch butterflies, migratory birds, salmon, bats, and mole rats all have the ability to navigate using Earth’s geomagnetism. But just what gives animals their magnetic sense has been a mystery. A new piece to the puzzle is a possible magnetoreceptor protein christened MagR and characterized by a team led by Can Xie of Peking University, although the mechanism behind MagR’s power to sense magnetism remains unknown (Nat. Mater. 2015, DOI: 10.1038/nmat4484). Researchers had already identified some cryptochrome proteins as light and magnetism sensors. But cryptochromes can only sense geomagnetic field inclination, which is the angle of the field relative to the equator. Something else is needed to sense north-south polarity. Xie and colleagues found that MagR forms rodlike complexes with cryptochromes and that the two proteins are coexpressed in pigeon retinas. The rod complexes contain 20 MagR proteins in the center surrounded by 10 cryptochrome proteins. Each MagR contains an iron-sulfur cluster, which lines up down the center of the rod in five loops of four clusters each. Crystals of the complexes line up with magnetic fields, and Xie and colleagues had to fashion crystal handling tools out of plastic because the crystals stuck to tools made with iron.