A patient who inherited two defective copies of a gene required by white blood cells, suffered from a severe immune deficiency disease. The patient recovered from the disease following gene therapy, in which he received an expressed copy of the 'missing' gene (the desired outcome). Unfortunately, he later developed leukaemia, a cancer involving the white blood cells (an unexpected outcome). The introduced gene was not responsible as it did not encode a carcinogen or a regulator of cell replication.
In a few sectences explain how might the transformation or DNA integration process have caused this outcome?