A family of proteins that shuttle metal ions to specific sites within a cell. The target sites for metal delivery include a number of metalloenzymes, or proteins that bind metal ions, such as copper, zinc, or iron, and use these ions as cofactors to carry out essential biochemical reactions. It was long thought that metalloenzymes could acquire their metal ion without the assistance of an accessory metallochaperone. The metal ion was believed to freely diffuse within the cell until it contacted its metalloenzyme target, at which point the metal ion inserted itself into the protein. However, several discoveries in the late 1990s led to a radical change in the thinking of how metalloenzymes acquire their metal ion. The insertion of the metal is now known to require an auxiliary protein, the metallochaperone. Metallochaperones escort the ion to a specific intracellular location and facilitate incorporation of the metal into designated metalloenzymes.