The famous two-faced cat, Frank and Louie (sometimes called Frankenlouie), passed away this week at the ripe old age of 15. The Massachusetts feline held the Guinness Book of World Records title for the longest lived Janus cat.

Named for the Roman god Janus, who was usually portrayed as having two faces, domestic cats with two faces are extremely rare, noted Leslie Lyons of the University of Missouri's Department of Veterinary Medicine and Surgery, who specializes in feline genetics. (Related: "R.I.P. Duecy: The Kitten With Two Faces.")

The animals also generally don't live very long due to health problems related to their deformity—making Frank and Louie's 15-year run that much more impressive, Lyons said.

The cause of Janus cats isn't completely understood, but "we know there's a variety of genetic mechanisms that could cause it," Lyons said. Only DNA testing can pinpoint the exact cause.

For instance, Janus cats may have too much of the sonic hedgehog (SHH) protein, which plays a role in forming an animal's face during development. In some experiments, chick embryos expsoed to an excess of SHH were born with two beaks and eyes spaced far apart.

Frank and Louie was a ragdoll cat, and while cats—both purebred and otherwise—can fall prey to a number of genetic problems, Lyons doesn't think breeding plays a role in the occurrence of Janus cats, since the condition is so rare.

Lucky Feline

Though it may have two faces, a Janus cat almost certainly shares one brain, Lyons said. (Related: "Venus the Two-Faced Cat a Mystery.")

"I would suspect there would be brain-function abnormalities."

Lyons says the fact that other organs like the esophagus weren't doubled up likely saved Frank and Louie's life and allowed it to survive. (Take a cat quiz.)

"In this particular case, one [side] didn't have a lower jaw or esophagus. If you had both sides of the face eating, I don't know what kind of complications that would cause. Initially the complications are going to be with the ability to eat and breathe properly."

Two-faced animals aren't limited to cats, either, she added.

"It can probably happen in most any mammal, vertebrate, [or] marsupial," said Lyons.

Compassionate Owner

Although Janus cats often die young, when left to their own devices cat mothers will often abandon, kill, or even eat a kitten with such severe deformities.

Frank and Louie's owner Marty Stevens has said in previous interviews that she took the cat home so that it wouldn't be euthanized, something Lyons applauds. (Watch National Geographic cat videos.)

"It's very nice that there are individuals in our society that have this type of compassion," said Lyons.

"Like taking care of a handicapped child, they've taken care of a [special-needs] cat for 15 years."