Biomass Mutation — ruling 1

Choosing 0 as the value for X will likely cause creatures you control to become 0/0 and be put into the graveyard.

Biomass Mutation — ruling 2

Biomass Mutation overwrites any effects that set the power and/or toughness of a creature you control to a specific value. Effects that modify power and/or toughness but don't set them to a specific value (like the one created by Giant Growth), power/toughness changes from counters, and effects that switch a creature's power and toughness will continue to apply. For example, say you control a 1/1 creature that's getting +4/+2 from an Aura, has a +1/+1 counter on it, and is affected by an effect that switches its power and toughness. This creature is 4/6. If you cast Biomass Mutation with X equal to 4, the creature would become 7/9 (4/4, +4/+2 from the Aura, +1/+1 from the counter, then switch power and toughness).

Biomass Mutation : règles, fonctionnement et interactions (MTG)

Biomass Mutation is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Creatures you control have base power and toughness X/X until end of turn.

How it works : Biomass Mutation

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2013· Wizards of the Coast
Choosing 0 as the value for X will likely cause creatures you control to become 0/0 and be put into the graveyard.
No other card is named in this ruling.
#2January 24, 2013· Wizards of the Coast
Biomass Mutation overwrites any effects that set the power and/or toughness of a creature you control to a specific value. Effects that modify power and/or toughness but don't set them to a specific value (like the one created by Giant Growth), power/toughness changes from counters, and effects that switch a creature's power and toughness will continue to apply. For example, say you control a 1/1 creature that's getting +4/+2 from an Aura, has a +1/+1 counter on it, and is affected by an effect that switches its power and toughness. This creature is 4/6. If you cast Biomass Mutation with X equal to 4, the creature would become 7/9 (4/4, +4/+2 from the Aura, +1/+1 from the counter, then switch power and toughness).
No other card is named in this ruling.