text: In biological systems, only manganese (Mn) is readily capable of replacing Mg, but only in a limited set of circumstances. Mn is very similar to Mg in terms of its chemical properties, including inner and outer shell complexation. Mn effectively binds ATP and allows hydrolysis of the energy molecule by most ATPases. Mn can also replace Mg as the activating ion for a number of Mg-dependent enzymes, although some enzyme activity is usually lost. Sometimes such enzyme metal preferences vary among closely related species: For example, the reverse transcriptase enzyme of lentiviruses like HIV, SIV and FIV is typically dependent on Mg, whereas the analogous enzyme for other retroviruses prefers Mn.