The FCK syndrome (also called "Flat Puppy Syndrome") is a congenital malformation of the chest that occurs in various mammals and usually appears 2 - 5 days after birth. Not only breeders, but also veterinarians often have to watch helplessly as their little protégés suffer, because despite intensive research at universities and veterinary schools, the cause is still largely unknown.
Since there are already various sources with detailed literature on the FCKS on the web, I limit myself here to the most important key data.
4-6 days after birth, the flattened ribcage can be felt by palpation (thorax drawing 2). The last degree of FCK syndrome is a deep furrow on the breastbone, which extends inwards and compresses the heart and lungs so that the kitten / puppy is decreasingly oxygenated.