Santa's Little Helper and is the center of the plots of several episodes. During the course of the show, he has fathered litters of puppies, passed obedience school, had surgery for bloat, replaced Duffman as the mascot for Duff Beer, and been trained as a police dog at Springfield's Animal Police Academy. Some of the episodes that focus on Santa's Little Helper have been inspired by popular culture or real experiences that staff members of the show have gone through. Although cartoon animals are often anthropomorphized, Santa's Little Helper generally exhibits canine behavior. Cast member Dan Castellaneta currently provides the dog's sounds on
Helper dependent virus and capsid encoding DNA, plus helper DNA. Every cell that is successfully transfected with all three DNA fragments will produce the necessary proteins to produce infective viruses. These viruses will only have transgenic DNA encapsidated and therefore once they've infected a patient's cell, they will not be capable of reproducing. Helper dependent viruses can also occur in nature without being "gutted". The term satellite virus has been given to a large group of viruses that all require the presence of another virus to replicate. Many of these are plant viruses, but animal viruses can be seen in the case of