In the case of helper-dependent viral vectors, what does the helper virus contain?

In the case of helper-dependent viral vectors, what does the helper virus contain? Correct Answer Missing genes

If the insertion of transgene replaces an essential viral gene, the vector is rendered helper dependent. A helper virus is then co-introduced which carries the missing genes.

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The retroviral transfected DNA is infected into mammalian cells by helper retrovirus. Helper retrovirus should not be packaged and the method adopted for this is?