There are 2 options: Original Kit (Sizes 0-5) and the Expanded Kit (Sizes 0-8). It is very important to measure your puppy using the sizing chart to determine which kit you will need. Puppies starting on a size 4 should purchase the Expanded (0-8 kit). After almost losing one of our puppies to flat chest, we created a cast to help treat the condition. Pectus Excavatum is an extremely dangerous condition in which a puppy is born with or develops a flat chest due to environmental factors. Luckily, there is a solution. Our kits come with several casts that your puppy can grow into, as needed. Please measure your puppy with our sizing chart prior to making a purchase! It's always important to consult with your veterinarian, as a flat chested puppy could have developed pneumonia or other respiratory issues through aspirations or other stress on the lungs. Please note, this kit has been designed around French Bulldogs, but has been successful in treating other breeds & even kittens. Feel free to reach out with any questions!