Dungeons and Dragons: After the Fall
Every 4 levels you gain a minor mutation. (4/8/12/16/20)
Every 5 levels you gain a major mutation. (5/10/15/20)
For every day you spend in a radiated area there is a cumulative 5% chance of gaining a random mutation. This maxes out at 50% and resets when you exit the area or gain a mutation. There is a 25% chance of a negative mutation.
If you roll a 1 on a save against radiation you gain a negative mutation. (1-90-minor, 91-100-major)
If you roll a 20 on a save against radiation you gain a positive mutation. (1-75-minor, 76-100-major)
Some spells can trigger mutations.