The protein corona is the layer of adsorbed proteins that forms on a nanoparticle once it enters a biological fluid ^[1]. Its composition is set by the abundance of the proteins, by their affinity for the surface and by competitive exchange between them, so the layer evolves from a loosely bound initial state to a more compact and longer-lived one ^[1]. Surface charge, hydrophobicity and particle curvature decide which proteins bind and how tightly ^[2]. The corona replaces the engineered surface as the interface that cells encounter, so the biological identity of the particle is the corona rather than the bare material ^[3]. Because composition depends on the fluid and on exposure time, the corona is described together with the medium and the incubation period ^[2].