A three$-$year$-$old boy was being treated by gene therapy for SCID using a modified retrovirus, but the boy developed leukemia. Several other children being treated this way also developed leukemia.$$ What was the cause of this?

Group of answer choices

Cas$9$ cut the DNA strand in the wrong spot of only one strand.

The disorder is known to cause leukemia in most patients.

The retrovirus vector triggered an immune response.

The gene retrovirus vector inserted into an oncogene and this activated leukemia.