text: If enough -1/-1 counters are put on a creature you control at the same time to make its toughness 0 or less (or the damage marked on it to be lethal), the number of +1/+1 counters on it before it got any -1/-1 counters will be used to determine how many Mutagen tokens you get. For example, if there are three +1/+1 counters on a creature and it gets four -1/-1 counters, you'll get three Mutagen tokens. That's because The Ooze's triggered ability checks the creature's existence just before it leaves the battlefield, and it still has all those counters on it at that point.