Darksteel Mutation

• WWizards of the Coast2013-10-17
  Official Ruling
  Darksteel Mutation overwrites the printed power and toughness of the enchanted creature, as well as any characteristic-defining abilities that define power and/or toughness.
• WWizards of the Coast2013-10-17
  Official Ruling
  Darksteel Mutation doesn't affect the enchanted creature's colors, if any. It will continue to be whatever color or colors it was before Darksteel Mutation entered the battlefield.
• WWizards of the Coast2013-10-17
  Official Ruling
  However, Darksteel Mutation does not overwrite effects that change the enchanted creature's power or toughness without setting it to a specific value (such as the ones created by Giant Growth or Glorious Anthem). It also won't overwrite the effect of counters.
• WWizards of the Coast2013-10-17
  Official Ruling
  The enchanted creature will be only an artifact and a creature, not any other card types. It will be only an Insect, not any other creature types.
• WWizards of the Coast2013-10-17
  Official Ruling
  Darksteel Mutation causes the enchanted creature to lose all abilities except indestructible at the time it becomes enchanted. Any abilities the creature gains after that point will work normally.
• WWizards of the Coast2013-10-17
  Official Ruling
  The creature will keep any supertypes it previously had. Notably, if Darksteel Mutation is enchanting a legendary creature, that creature will continue to be legendary. Also, if it's enchanting a commander, that creature will continue to be a commander.
• WWizards of the Coast2013-10-17
  Official Ruling
  In some rare cases, the creature may have subtypes other than creature types before becoming enchanted with Darksteel Mutation. If it had any other artifact subtypes (such as Equipment), it will retain those. If it had any subtypes other than artifact types and creature types (such as Shrine), it won't retain those.
• WWizards of the Coast2013-10-17
  Official Ruling
  Darksteel Mutation overwrites any previous effects that set the enchanted creature's power or toughness to a specific value. Any such effects that start to apply after Darksteel Mutation entered the battlefield will work normally.

Rulings via Scryfall