A compound is a small square of land that you have to place on your rooftop so you can put mutants on it.

A mutant can only be placed on a compound that matches its gene. For example, a mutant that has a Necro gene can only be placed on a Necro compound. If your mutant is a double gene, for instance Necro-warrior, you will be able to place it whether on a Necro compound or a Warrior compound.

Note that compounds have different sizes, allowing you to place up to 4 creatures on it.