Mutation resistance

Version 0.18: This article may not be up to date for the latest stable release of Crawl.

Mutation resistance (rMut) provides player a chance to avoid being mutated. Different sources of mutation resistance generally stack additively.

Sources

You may acquire the Mutation Resistance mutation. It has three ranks: at ranks 1 and 2, 66% of attempts to mutate you will be blocked. Additionally, at rank 2, 50% of attempts to remove mutations with a potion of cure mutation will fail. At rank 3, you can neither add nor remove any more mutations: they are fixed. The only way to cure them is to use Zin's one-time ability to cure all mutations. Halflings start with this mutation at level 1.
Two unrands, the Hat of the Alchemist and the Lajatang of Order, provide mutation resistance when equipped. This is a 66% chance, equivalent to the first rank of the Mutation Resistance 1 mutation. Using both at the same time won't increase the resistance.
Zin provides a (piety/200)% chance to protect against attempts to mutate you. Maxed out piety is a perfect defense against mutation attacks.
Being undead makes it so that you will rot instead of mutate: in general, this is easier to cure than mutations. Vampires don't function as undead in this aspect, and will mutate normally. Additionally, Necromutation causes you to become effectively undead, and so can be used as an emergency source of mutation resistance if you're prepared to deal with rotting.

Beyond that, your existing mutations also provide mutation resistance of a sort.

Each mutation you already have gives you 6.5% resistance against future random mutations. This ONLY applies to random mutations; good mutations gained through potions of beneficial mutation, or through use of the Helix card at high power, as well as bad mutations from enemy attacks, still affect you normally.
Any time you resist a random mutation through this method, you have a 33% chance of losing one of your mutations as well. This allows you to use potions of mutation as limited potions of cure mutation if you find yourself suffering from many, many bad mutations, though it is by no means guaranteed not to simply make things worse.

Prior to 0.15, vampire mutation effects depended on their satiation level.