Mutate
from the start, and no skill slot is required for it.
However, this is not an action which should be used anywhere near the start of the game, as it is irreversible, and will reveal a player as 
Mush to everyone. In fact, using it too early is a common beginner's mistake.
Upon mutating, the mush player will turn into a Mush Berserker, making them drop all of their items and lose all of their skills (maybe not the passive Mush skills), as well as some standard actions such as Extract a Spore and Infect. He will also be unable to pick up new items, or speak on the public channel.
TODO: Confirmation needed
In turn, mutating will 
heal the player, and give a bonus to unarmed attacks, as well as some damage protection, comparable to a player having 
Solid and wearing the 
Plastenite Armor (possibility of "armor blocked damage" combat events).
When Mutate is used to engage in open battle, it is often as a last resort, and can quickly determine how the game will end.
With full , a mutated Mush can easily kill a healthy human player (however you must be aware of Ian's Pacifist trait).
For better results, several Mush players can arrange a set time for mutating simultaneously as a pack. Though even then, it is usually more effective to use Weapons. It does make for a good distraction, though...
At times this action can be unavailable. The exact circumstances that cause this are still unknown. The screenshot on the right was taken by a mush player who had eaten a Cooked Ration after the humans had completed the Constipaspore Serum Research.
