Santa's Little Helper event; however, their plan fails and a drunk shark that's discovered at the event becomes the new mascot instead. Santa's Little Helper gladly returns to the Simpson family. Santa's Little Helper's initial appearance on "The Simpsons" was in the first episode of the series, "Simpsons Roasting on an Open Fire", which aired on Dec. 17, 1989. Since then, he has become a recurring character. "The Simpsons" creator Matt Groening told "TV Guide" in 2000 that "we [the staff] painted ourselves into a corner with our Christmas episode. Once we wrote the dog into the show, we were stuck with him."
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