Santa's Little Helper of their sacrifices and starts treating him badly, causing him to run away. He ends up in the possession of Mr. Burns, who trains him to become a vicious attack dog. Several days later, Bart stumbles upon the trained Santa's Little Helper and is attacked, but the greyhound eventually recognizes his old friend and stops. In "The Canine Mutiny" (season eight, 1997), Bart uses a fake charge card to buy a well-trained rough collie named Laddie from a mail-order catalog. Laddie learns many tricks that Santa's Little Helper is completely unable to perform, and the Simpson family nearly forgets about
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