The effect desired is expression of a wild type gene to correct the disease confered by the patient's mutated gene. Ostensibly insertion anywhere that results in expression of the delivered gene is all that is needed.
The side effect in this case is the insertion near LMO2. Retroviruses tend to insert near oncogenes, especially those related to leukemia, such as LMO2.
There definitely are multiple insertions. How many and their randomness is unknown. Only some will effect a cell -- if the insertion is near a gene that contols proliferation, for example, the proximal integration of the virus elements can up regulate that gene compared to a normal cell. This can cause a cancer, or it could actually be benign and perhaps even help to increase the number of cells expressing the therapeutic gene introduced by the vector. In such as case insertion near such a gene can increase the efficacy of the gene therapy treatment.
There's a lot unknown still in the whole broad field, from the basic molecular and cellular biology to the application of it for medical purposes.
How many of thes insertions were in germ cells that produced children?