Arrange the following steps of gene therapy for ADA deficiency in the correct sequence.
I) Infection of lymphocytes by genetically engineered retrovirus
II) Culture of lymphocytes
III) Isolation of lymphocytes from ADA deficient patient
IV) Genetically modified lymphocytes are injected back to patient

The correct option is B III, I, II, IV
SCID (Severe Combined Immunodeficiency) is caused due to the deletion of the gene for the enzyme adenosine deaminase (ADA).

The ADA enzyme is crucial for the immune system to function. Lymphocytes are important cells of the immune system and help protect the body from infections. The patients with ADA deficiency have non-functional lymphocytes and so they cannot mount immune responses against invading pathogens.

In order to treat adenosine deaminase deficiency, gene therapy is used widely.

A proper, functional ADA gene needs to be inserted into the lymphocytes of the patient.

For this purpose, specifically designed harmless/disarmed vectors like retrovirus are used.

Lymphocytes are collected from the patient (III) and are allowed to be infected by retroviral vectors to get the normal ADA cDNA inserted into their genome. (I)

The genetically modified lymphocytes with the functional ADA gene are then cultured (II) and allowed to grow in the laboratory after which they are infused back into the patient.(IV)