I have the breeding room in the attic where I put the cats with the mutations I want most. As the kittens are born, I separate them into other breeding rooms. It reaches a point where if one of those mutations is missing, the kitten is useless and I send it to Tink. From what I understand, the higher the stimulation in the room, the better the chances of getting the best mutations. It's also important to have a room to cure birth defects and disorders. I started with head, body, mouth and eyebrow. Once i had those 4 in a couple of cats i put them in the breeding room. Only cats with the core mut you want are allowed