The Danger Room

Mutants you control get +1/+1 for each training counter on The Danger Room.
{3}{t}: Put a training counter on The Danger Room. If there are 3 or more training counters on The Danger Room, remove one.
{t}: Add one mana of any color to your mana pool. Spend this mana only to cast Mutant spells or activate abilities of Mutants.