A helper dependent virus also termed a gutless virus is a synthetic viral vector

Viral vector

Viral vectors are a tool commonly used by molecular biologists to deliver genetic material into cells. This process can be performed inside a living organism or in cell culture . Viruses have evolved specialized molecular mechanisms to efficiently transport their genomes inside the cells they infect...

 dependent on the assistance of a helper virus

Helper virus

A helper virus is a virus used when producing copies of a helper dependent viral vector which does not have the ability to replicate on its own. The helper virus is used to coinfect cells alongside the viral vector and provides the necessary enzymes for replication of the genome of the viral vector....

 in order to replicate.
Since the genome of the gutless virus does not include genes encoding the enzymes required to replicate, it is deemed safe for use in gene therapy

Gene therapy

Gene therapy is the insertion, alteration, or removal of genes within an individual's cells and biological tissues to treat disease. It is a technique for correcting defective genes that are responsible for disease development...

since an infection cannot occur except in the presence of a suitable helper virus.