For most animals, humans and kittens included, a bum is considered fairly important.
But for poor little Onion, dumped on London's streets at the tender age of just nine weeks, her genes decided it was more optional than compulsory.
Once rescued, it was discovered she suffered from a condition called Atrensi ani - which means she doesn't have a bum.
"Instead, her intestinal tract ends in her vulva, through which she is pooing as well, but with great difficulty," her foster parents wrote on crowdfunding site JustGiving.
"As a complication she has also developed a nasty bacterial infection in her urinary tract."
Normally a kitten which such a complication would be euthanised, but as Onion's condition has improved the vets are desperate to give her a second chance at life.
And thanks to a public outpouring of support, Onion is getting her new bum - "a bum that will make a Kardashian jealous", her foster parents say.
Her surgery is scheduled for November 29, by vets at the Royal Vet College's Queen Mother Hospital for Animals.